Fight blows up (4)
I believe the answer is:
spar
'fight' is the definition.
(I know that spar is a more specific form of the action box)
'blows up' is the wordplay.
'blows' becomes 'raps' (rap is a kind of blow).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'raps' back-to-front is 'SPAR'.
(Other definitions for spar that I've seen before include "dispute" , "Go a few rounds with the ship's pole" , "microcrystalline mineral?" , "Mineral; plane's wing beam" , "Box (gently) - fight (verbally)" .)