Crossword Genius

Knocks up a fight (4)

Ross

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)'" .)

I've seen this clue in The Telegraph.
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