Knocks over in fight (4)
I believe the answer is:
spar
'fight' is the definition.
(sparring is a kind of fighting)
'knocks over' is the wordplay.
'knocks' becomes 'raps' ('rap' can be a synonym of 'knock').
'over' says the letters should be written backwards.
'raps' reversed gives 'SPAR'.
'in' acts as a link.
(Other definitions for spar that I've seen before include "Mineral; strong pole" , "A mineral; nautical pole" , "Box (gently) - fight (verbally)" , "Yard" , "Mineral; box; wing beam" .)