Bar bird (4)
I believe the answer is:
rail
I believe this is a double definition.
'bar' is the first definition.
(rail is a kind of bar)
'bird' is the second definition.
(rail is a kind of bird)
(Other definitions for rail that I've seen before include "Part of train track" , "Scoff" , "Protest strongly (at)" , "Bar for hanging; bird as corncrake" , "Bar of rolled steel" .)