Birds in bars (5)
I believe the answer is:
rails
I believe this is a double definition.
'birds' is the first definition.
(rail is a kind of bird)
'bars' is the second definition.
(rail is a kind of bar)
'in' acts as a link.
(Other definitions for rails that I've seen before include "Water birds" , "For trains" , "Train tracks" , "Barriers of horizontal bars and supports" , "Wading birds - dysfunctional person has come off them" .)