The setter's sad, avoiding end of cruciverbal fill (5)
I believe the answer is:
imbue
'fill' is the definition.
(imbuing is a kind of filling)
'the setter's sad avoiding end of cruciverbal' is the wordplay.
I cannot quite understand how this works, but
'setter's' could be 'im' (I've seen this in other clues) and 'im' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for imbue that I've seen before include "Permeate, fill" , "Permeate - inspire" , "Charge" , "Inspire (with ideals)" , "dye" .)