Whack, as a fly (4)
I believe the answer is:
swat
'whack as a fly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for swat that I've seen before include "Hit a wasp or fly" , "Hit and crush (a fly)" , "deliver blow" , "land crushing blow" , "Kill (fly)" .)