Whack (4)
I believe the answer is:
swat
'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 "strike smartly" , "Hit smartly" , "Deal a quick and effective blow (to)" , "Hit a wasp or fly" , "Strike sharply (at fly?)" .)