Knocks back a boom (4)
I believe the answer is:
spar
'a boom' is the definition.
(I've seen this before)
'knocks back' is the wordplay.
'knocks' becomes 'raps' ('rap' can be a synonym of 'knock').
'back' is a reversal indicator.
'raps' reversed gives 'SPAR'.
(Other definitions for spar that I've seen before include "Scrap" , "What boxers do" , "Engage in argument" , "microcrystalline mineral?" , "Fight - something probably rigged?" .)