Heartless benefactor's opportunity (4)
I believe the answer is:
door
'opportunity' is the definition.
(I've seen this in another clue)
'heartless benefactor's' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'benefactor' becomes 'donor' (donor is a kind of benefactor).
'donor' with its centre taken out is 'DOOR'.
(Other definitions for door that I've seen before include "Opener" , "Entry point" , "Alternative to knocker" , "place of entry" , "See 1" .)