Former partner works to return bonus (5)
I believe the answer is:
extra
'bonus' is the definition.
(thesaurus)
'former partner works to return' is the wordplay.
'former partner' becomes 'ex'.
'works' becomes 'art' (I've seen this before).
'to return' says the letters should be written backwards.
'art' reversed gives 'tra'.
'ex'+'tra'='EXTRA'
(Other definitions for extra that I've seen before include "walk-on part" , "An increased amount" , "(Theatrical) spear carrier" , "Something additional or bit player in movie" , "more than usually" .)