A gratuity at the end (3)
I believe the answer is:
tip
I believe this clue is a double definition.
'a gratuity' is the first definition.
(as in a gratuity to a waiter)
'the end' is the second definition.
(tip is a kind of end)
'at' acts as a link.
(Other definitions for tip that I've seen before include "Point; sweetener" , "Reward service; end" , "Gratuity or light touch" , "Hint or dumping ground" , "End; overbalance" .)