"As a bonus . . ." (4)
I believe the answer is:
also
'as a bonus' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for also that I've seen before include "As well, in addition" , "item's rarely used now for this" , "an indication of more to come" , "in accompaniment" , "To boot" .)