Sign for something extra (4)
I believe the answer is:
plus
I believe this clue is a double definition.
'sign' is the first definition.
(plus sign is a kind of sign)
'something extra' is the second definition.
(I've seen this in another clue)
'for' acts as a link.
(Other definitions for plus that I've seen before include "Positive and not a negative" , "Advantage; also" , "sign of the cross" , "An advantage; positive" , "mark for arithmetic" .)