This clunk build is designed to be fairly versatile, although it relies on clunk's base health entirely, using bite and med-i-can to return health. Build order is very flexible, depending on what you feel is most useful to your teammates - sometimes ramping the damage on Explode first is the best thing to do, but sometimes you want to just stop killing yourself, with a Titanium Hardhat. Snare bite is a must at some point in this build, but if your teammates refuse to work with you, you might need some damage (in the form of Explode) before you buy the snare.
Missiles: Personal preference, generally including Salvo Value Pack. Improved Homing Sensor can be good for getting in some early game damage with easy hitting missiles, but that's an entirely different build (and skill may allow you to hit most missile shots anyway!)
My initial purchase order is fixed: Piggy Bank, Space Air Max, Vacuum Bite and 1 level of Med-i'-can.
After here, what I buy depends on my team and how much solar I have when I back. I try to buy something every time I back, so that when I return to the fight, I have some sort of further advantage.
High Priority items: 2nd level of Med-i'-can, Screamer Engine, Explode.
For more lane sustain: Power Converter and The Suckanator Power 9000 Cleaner. It is important to buy power converter first, as suckanator with no power converter will remove 1/3 of your bite damage! Bite damage is pretty useful. If your teammates aren't helping you, you probably want to take Explode, but if you are with teammates, and trying to tank damage for them, this combo can be very potent, healing you for 64 health per bite, on top of your med-i'-can heals.
For more damage: Find yourself surrounded by opponents? Scared to use explode due to that 40 damage bite back? What you need is Thermonuclear Cleaner and Titanium Hard Hat. The order in which you buy them is up to you, but I tend to go hard hat 1, cleaner 1, hard hat 2. After this point I usually want to spend solar somewhere else (especially on cooldown reduction - Reactor Cooler), but 100 damage explode is nothing to sniff at, so the second cleaner upgrade will also be worth the buy if you happen to go back with 215 solar.
The first tip is one you'll hear everywhere. Some call it 'Ninja Clunk.' Basically it's when you start your explosion in the sky, and utilise the incredibly swift vertical motion to drop down into where the enemy are (especially by their turret - remember that explode damages turrets), dealing surprise heavy damage that they never saw coming! Map awareness obviously helps here, so that you know your opponents are going to be there when you fall. Using a bite as you land can also be very effective, maximising your damage on one individual - this is one of the ways that clunk can rack up the kills. Obviously the more savvy opponents will be watching out for this, especially after the first time, but it can still be fairly successful.
Secondly is bite - spam it. Make sure you hit. You lost a bit of health? Bite a droid. Gone to the jungle to restore health? Bite a creep. Restoring health allows you to stay in lane, and soak up a bit of damage for your teammates. Remember while taking damage, not to die! Also remember that needlessly taking damage (that your whole team could just dodge and avoid) is not necessary (clearly). Contradictory to this suggestion, also make sure that you have bite ready when you need it, but its short cooldown means that this isn't usually an issue.
Missiles - spam them. Even when the opponent is jumping - missiles have a slow enough speed that the opponent may well fall right back down on your missile. Don't hold the button, click every now and then, as holding reduces your movement speed. hitting with missiles (even unupgraded) deals 14 damage still, which is very nice, and really racks up over 3 or 4 hits. So basically, don't forget to use your missiles!
Turrets - Tank them, but only when you're sure the enemy team isn't about to come up from behind and stunlock you, and chuck huge burst damage at you. Works best as a team - clunk has enough health to tank a lot of damage from the turret, and the droids do a fair amount of damage themselves. Bite enemy droids, maybe even explode them, keep your push going as long as possible (but always be wary of opponents - needlessly dying is never a good thing). Clunk is very strong at taking down turrets in this way, especially when teamed up with someone with good DPS (Derpl, Lonestar, even melee Coco, and Leon builds. Shotgun froggy is also particularly effective. Ok so I just listed half the 'nauts, and others will also fit! Basically just team up with someone!)
Jump. A lot. Ever been annoyed at people jumping around avoiding your attacks? Well you can do it too! Even as Clunk! Tap your jump, don't hold it, and you can jump further and higher than ever before. Jump backwards and forwards, try not to be too predictable. You can even juke fast 'nauts like froggy in this way (although getting out of position can be deadly for Clunk, due to his slow speed, so don't be too cocky). When charging Explode, try to jump over and around enemies, so that their escape route changes suddenly, and is out of their control.
Explode - Some more explode tips. Explode can hit the base when you are standing on the ledge above it, so you can use this when people are trying to turtle - although be aware that they can always just move out of the base and come and get you! Vertical movement is the fastest, so exploding will be less useful around glass platforms, which your opponents may suddenly drop through and leave your explosion zone. Remember that you can use your explode as a scare tactic - people instinctively want to run away from you when you're exploding, but also remember that you can kill yourself!
So to sum up, ninja drop from above, push turrets hard, stay healed up using your bite, and abuse the platforming mechanics of the game to avoid enemy attacks (Another thing to do would be try and position yourself so that the opponent can't attack you without repositioning themselves. On the other side of a solid platform for example (e.g. centre of top lane on Sorona, although this works best against nauts with low, slow jumps like derpl/lonestar rather than fast, high jumping nauts like froggy, who can reposition easily).
If you have any additions or objections, feel free to post them somewhere (in clunk's comments maybe?) or even post your own clunk build. Clunk is one of the nauts I have had the most success with, which is why I hope this build guide will be useful. At the same time I acknowledge that I'm not perfect, and that several people out there are better at clunk than I am!
Hope this build helped, Good luck and have fun with all your awesome endeavours (and sorry for the wall of text, hope you made it through all right).