Something sticky in pie, unfinished (3)
I believe the answer is:
tar
'something sticky' is the definition.
'pie unfinished' is the wordplay.
'pie' becomes 'tart' (I've seen this before).
'unfinished' means to remove the last letter.
'tart' with its last letter taken away is 'TAR'.
'in' acts as a link.
(Other definitions for tar that I've seen before include "Flammable liquid; sailor" , "Dark viscid substance" , "Old salt or liquid from coal" , "used by those who pay to work on deck" , "Sailor; road surface" .)