Green’s Dictionary of Slang

a hit, a blow.

1900		Marvel 15 May 15: I got a clapper on the nut wid a few dead pigs.
1994		(con. 1930s–40s) K.C. Kearns Dublin Tenement Life 116: He gave me the baby – and I’ll never forget it – he said, ‘Give it a few clappers on the bottom’.