Beer garden's final wader (7)
I believe the answer is:
bittern
'wader' is the definition.
(wading bird)
'beer garden's final' is the wordplay.
'beer' becomes 'bitter' (bitter is a type of beer).
'final' says to take the final letters.
The last letter of 'garden' is 'n'.
'bitter'+'n'='BITTERN'
(Other definitions for bittern that I've seen before include "Bird of marshes" , "Bird with a booming cry" , "large wader" , "inhabitant of marsh" , "Marsh bird of the heron family" .)