Green’s Dictionary of Slang

1. (US) a dollar.

1907		C. M’Govern By Bolo and Krag 40: He having doled out the last clacker saved up in an old sock.
1919		Lima (OH) News 5 June 6/3: Coins are called [...] ‘clackers’.
1929		A.J. Barr Let Tomorrow Come 42: He sends me in some Durham an’ a few clackers.

2. a foreign coin of low value.

1929		L. Thomas Woodfill of the Regulars 58: They would sell it to us for two ‘clackers,’ or two Spanish cents, a glass.
1932		B.M. Harvey Me and Bad Eye and Slim 103: You give her a clacker (two cent piece) and she hands you four pieces of paper and opens the door.