A wading bird in a bar (4)
I believe the answer is:
rail
I believe this clue is a double definition.
'a wading bird' is the first definition.
(rail is a kind of wading bird)
'bar' is the second definition.
(I know that rail is a type of bar)
'in a' is the link.
(Other definitions for rail that I've seen before include "Track for wheels" , "transport system" , "Bar of track or fence" , "Complain bitterly - wading bird" , "Bird with long legs" .)