Box blows up (4)
I believe the answer is:
spar
'box' is the definition.
(sparring is a kind of boxing)
'blows up' is the wordplay.
'blows' becomes 'raps' (rap is a kind of blow).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'raps' backwards is 'SPAR'.
(Other definitions for spar that I've seen before include "Box (lightly) - fight (verbally)" , "Fight - something probably rigged?" , "What boxers do" , "Crystal - box" , "Act as boxer" .)