Part of tennis match repeatedly knocked back balls (6)
I believe the answer is:
'balls' is the definition.
(balls can informally mean testicles)
'part of tennis match repeatedly knocked back' is the wordplay.
'part of tennis match' becomes 'set' (a tennis match is made up of sets).
'repeatedly' becomes 'set' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'knocked back' shows that the letters should be reversed in order.
'setset' written backwards gives 'TESTES'.
(Another definition for testes that I've seen is " balls".)