Box blows over (4)
I believe the answer is:
spar
'box' is the definition.
(sparring is a kind of boxing)
'blows over' is the wordplay.
'blows' becomes 'raps' (rap is a kind of blow).
'over' is a reversal indicator.
'raps' backwards is 'SPAR'.
(Other definitions for spar that I've seen before include "Mineral; plane's wing beam" , "Mineral; box; wing beam" , "Practise boxing with a pole?" , "Raps with large beam" , "have an argument without marked hostility" .)