Knocks box over (4)
I believe the answer is:
raps
'knocks' is the definition.
('rap' can be a synonym of 'knock')
'box over' is the wordplay.
'box' becomes 'spar' (spar can mean to box).
'over' says the letters should be written backwards.
'spar' backwards is 'RAPS'.
(Other definitions for raps that I've seen before include "'Knocks, on the door say (4)'" , "Sprechgesang pop hits" , "Cracks" , "Knocks sharply" .)