Knocks up a fight (4)
I believe the answer is:
raps
'knocks' is the definition.
('rap' can be a synonym of 'knock')
'up a fight' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'a fight' becomes 'spar' (I've seen this before).
'spar' reversed gives 'RAPS'.
(Other definitions for raps that I've seen before include "Sprechgesang pop hits" , "Knocks sharply" , "Cracks" , "'Knocks, on the door say (4)'" .)