Whack (4)
(Other definitions for sock that I've seen before include "One going into Oxford maybe" , "Hit; horse's leg marking" , "Very short stocking?" , "impact" , "Hit with a powerful blow" .)
'whack' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for swat that I've seen before include "Crush (eg, a fly)" , "deliver blow" , "Hit a wasp or fly" , "Strike sharply (at fly?)" , "'Twas quite a smack" .)