Return the favour to swimmer retaining record (5)
I believe the answer is:
repay
'return the favour' is the definition.
'repay' can be an answer for 'return' ('repay' can be a synonym of 'return'). I'm not sure about the 'the favour' bit.
'swimmer retaining record' is the wordplay.
'swimmer' becomes 'ray' (ray is a kind of aquatic vertebrate).
'retaining' means one lot of letters goes inside another.
'record' becomes 'ep'.
'ray' enclosing 'ep' is 'REPAY'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for repay that I've seen before include "Requite" , "recompense" , "Do or give in return" , "Quite old" , "Refund or reward" .)