Knight leaves wading bird to get a drink (6)
I believe the answer is:
bitter
'a drink' is the definition.
(I know that bitter is a type of pale ale)
'knight leaves wading bird' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'leaves' is a deletion indicator.
'wading bird' becomes 'bittern' (bittern is a type of wading bird).
'bittern' with 'n' removed is 'BITTER'.
'to get' acts as a link.
(Other definitions for bitter that I've seen before include "Hard to accept" , "Beer; not sweet" , "Sour British beer?" , "Resentful about English beer?" , "Causing unhappiness" .)