Knocks spar about (4)
I believe the answer is:
raps
'knocks' is the definition.
('rap' can be a synonym of 'knock')
'spar about' is the wordplay.
'about' shows that the letters should be reversed in order.
'spar' back-to-front is 'RAPS'.
(Other definitions for raps that I've seen before include "Cracks" , "Knocks sharply" , "'Knocks, on the door say (4)'" , "Sprechgesang pop hits" .)