Setter’s left — left unhappy with fill (5)
I believe the answer is:
imbue
'fill' is the definition.
(imbuing is a kind of filling)
'setter's left left unhappy' is the wordplay.
'setter's' becomes 'im' (referring to the person who set the crossword).
'left' becomes 'l' (common abbreviation).
'left' suggests deleting specific letters.
'unhappy' becomes 'blue' (I've seen this before).
'blue' with 'l' taken away is 'bue'.
'im'+'bue'='IMBUE'
'with' acts as a link.
(Other definitions for imbue that I've seen before include "Endow" , "Fill with, infuse" , "Penetrate" , "Spread or diffuse through; inspire" , "Permeate - inspire" .)