Something to do with interest (5)
I believe the answer is:
repay
'something to do with interest' is the definition.
I can't tell whether this definition defines the answer.
'to do with interest' is the wordplay.
'to do with' becomes 're' (regarding).
'interest' becomes 'pay' (I am not sure about this - if you are sure you should believe this answer much more).
're'+'pay'='REPAY'
Can you help me to learn more?
(Other definitions for repay that I've seen before include "Quite old" , "Settle a score" , "Return money borrowed" , "Refund or reward" , "recompense" .)