Former partner works returning bonus (5)
I believe the answer is:
extra
'bonus' is the definition.
(I've seen this before)
'former partner works returning' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'works' becomes 'art' (I've seen this before).
'returning' shows that the letters should be reversed in order.
'art' in reverse letter order is 'tra'.
'ex'+'tra'='EXTRA'
(Other definitions for extra that I've seen before include "wide? It could be" , "late edition" , "E.g. wide in cricket" , "Bonus; surplus" , "Not a standard item" .)