Drink a wading bird left unfinished (6)
I believe the answer is:
bitter
'drink' is the definition.
(bitter is a kind of drink)
'a wading bird left unfinished' is the wordplay.
'a wading bird' becomes 'bittern' (bittern is a kind of wading bird).
'left unfinished' means to remove the last letter.
'bittern' with its last letter taken off is 'BITTER'.
(Other definitions for bitter that I've seen before include "Kind of British beer" , "Resentful and cynical" , "severe" , "Act nervously" , "drink" .)