Reject rap defender (5,4)
I believe the answer is:
knock back
'reject' is the definition.
(I know that reject can be written as knock back)
'rap defender' is the wordplay.
'rap' becomes 'knock' (synonyms).
'defender' becomes 'back' (left-backs and right-backs are football defenders).
'knock'+'back'='KNOCK-BACK'
(Other definitions for knock back that I've seen before include "Turn down - drink" , "finish off drink quickly" , "Refuse" , "Refusal" , "Disappointing rejection" .)