Bonus added (4)
I believe the answer is:
plus
I believe this clue is a double definition.
'bonus' is the first definition.
(I know that bonus can be written as plus)
'added' is the second definition.
(I've seen this before)
(Other definitions for plus that I've seen before include "Positive sign" , "Positive thing" , "Positive - addition" , "asset" , "Advantage; also" .)