Knocks over box (4)
I believe the answer is:
spar
'box' is the definition.
(spar can mean to box)
'knocks over' is the wordplay.
'knocks' becomes 'raps' ('rap' can be a synonym of 'knock').
'over' says the letters should be written in reverse.
'raps' reversed gives 'SPAR'.
(Other definitions for spar that I've seen before include "quarrel" , "Practice boxing with wooden beam" , "Practise boxing with a pole?" , "possibly rigged" , "Box without punching hard" .)