The Middens gather waste. The Tinkers consume it. Where the Skulks scavenge raw materials from Nod's refuse, the Tinkers process those materials into something usable. They are metalworkers, artificers, repairers of broken things, and manufacturers of new things from old parts.
The Tinkers are predominantly Skulk, though humans and others with mechanical aptitude have been absorbed into the trade over centuries. They operate workshops in the Middens and the lower Warrens, producing everything from scarbines (scrap guns) to rebuilt Earth electronics, jury-rigged generators, Wire-compatible communication devices, and simple arcane instruments refurbished from salvaged components. A Tinker workshop smells like solder and ozone. Their output is cheap, unreliable by the standards of a proper engineer, and indispensable to the lower classes who cannot afford guild-rate equipment.
The Tinkers have no formal charter, no leadership hierarchy, and no political ambitions. They're a trade community whose influence comes from the fact that half of Nod's tools, weapons, and household goods passed through their hands at some point in their lifecycle. Destroying the Tinkers would not be difficult but replacing what they produce would be.