Erstwhile partner works to return bonus (5)
I believe the answer is:
extra
'bonus' is the definition.
(thesaurus)
'erstwhile partner works to return' is the wordplay.
'erstwhile' becomes 'ex'.
'partner works' becomes 'art' (I can't justify this - if you can you should give a lot more credence to this answer).
'to return' says the letters should be written backwards.
'art' backwards is 'tra'.
'ex'+'tra'='EXTRA'
Can you help me to learn more?
(Other definitions for extra that I've seen before include "comes to 16 [TEST MATCH]" , "Bolt-on" , "Not a standard item" , "on top" , "Minor actor in crowd scenes" .)