A king's emolument (4)
I believe the answer is:
perk
'emolument' is the definition.
(both can mean a benefit or payment)
'a king's' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'king' becomes 'K' (abbreviation for king in chess).
'per'+'k'='PERK'
(Other definitions for perk that I've seen before include "how to make coffee" , "extra privilege" , "A little extra" , "Informal goodie given on top of salary" , "Fringe benefit" .)