The only explanation I've found is that it comes from "nobby" being a slang word for stylish or showy, and the nickname arose because many Clarks added an e to their name. If this is the correct derivation Nobby should only apply to someone with the surname Clarke; but in practice it applies to both Clarks and Clarkes. Anyone got any other suggestion?
Clerks wrote for long hours with pens - this left them with lumps and bumps on their fingers where the pen rubbed.
Hence their fingers were 'knobby' . . .