Refund accepted on behalf of wastrel (9)
I believe the answer is:
reprobate
'wastrel' is the definition.
(thesaurus)
'refund accepted on behalf of' is the wordplay.
'refund' becomes 'rebate' (rebate is a kind of refund).
'accepted' means one lot of letters goes inside another.
'on behalf of' becomes 'pro'.
'rebate' enclosing 'pro' is 'REPROBATE'.
(Other definitions for reprobate that I've seen before include "Berate pro, being an unprincipled person" , "criminal" , "17 [WRONGDOER]" , "Rogue" , "base" .)