Sign for the bonus (4)
I believe the answer is:
plus
This clue is a double definition.
'sign' is the first definition.
(plus sign is a kind of sign)
'the bonus' is the second definition.
(thesaurus)
'for' is the link.
(Other definitions for plus that I've seen before include "sign of more to come" , "Together with; advantage" , "Coupled with" , "An advantage; positive" , "Positive and not a negative" .)