"Clacker" is a word used by William Gibson and Bruce Sterling in their collaborative steampunk novel The Difference Engine. A clacker is basically the steampunk analogue of a hacker (in the classical sense), a shrewd computer, er, Engine programmer and also a data miner. The Engines in the novel seem to be mostly used for data storage and retrieval rather than for the computations that Charles Babbage and Lady Lovelace envisioned for them.
In The Difference Engine, the steam-powered Engines are property of the British Crown, under the rule of the illustrious Prime Minister Lord Byron, and the clackers are their operators. As often happens with these things, the French have their own competing technology, the Ordinateurs, to rival the Engines. However, the operators of the French Ordinateurs weren't called "claqueurs", as Webby might have us believe. A pity, because that would have been such a cool name for them.
Clackers are presumably so named because of onomatopoeia. Just imagine how noisy and clacky a steam-powered computer operating on punch cards must be.