Green’s Dictionary of Slang

(orig. US) to hit hard.

1944		D. Runyon Runyon à la Carte 76: The beautiful picks up what is left of her lobster Newburg, plate and all, and clops Brogan on the pimple.
1966		P. Willmott Adolescent Boys of East London (1969) 88: If you couldn’t catch on quickly, they just clopped you round the earhole.