Green’s Dictionary of Slang

clam n.4

[var. on lam n.2 ]

a heavy blow.

[US]H. Gould Fort Apache, The Bronx 105: He grabbed Freddie’s wrist with his left hand, and cracked him on the temple with the blackjack. The side of Freddie’s head opened like a piece of rotten fruit [...] ‘That was some clam you laid on him,’ Corelli said.