Whack down (3)
I believe the answer is:
hew
'whack down' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hew that I've seen before include "Cut with blows" , "Chop; sounds like colour" , "Chop or hack at" , "Carve out of wood or stone" , "Chop (wood)" .)