Extra benefit (4)
'extra benefit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for perk that I've seen before include "Informal goodie given on top of salary" , "Tip" , "Incidental benefit from one's employment" , "Fringe benefit" , "extra privilege" .)
(Other definitions for plus that I've seen before include "Positive thing" , "being a little cross" , "Additional" , "And, arithmetically" , "sign of more to come" .)