Knock down for taking half a loaf! (5)
I believe the answer is:
floor
'knock down' is the definition.
('floor' can be a synonym of 'knock down')
'for taking half a loaf' is the wordplay.
'taking' indicates putting letters inside.
'half a' suggests halving the letters.
'loaf' cut in half is 'lo'.
'for' placed around 'lo' is 'FLOOR'.
(Other definitions for floor that I've seen before include "Puzzle" , "Bottom surface" , "American's story" , "Disconcert" , "Inside horizontal surface" .)