Birds on track (5)
I believe the answer is:
rails
I believe this clue is a double definition.
'birds' is the first definition.
(rail is a kind of bird)
'track' is the second definition.
(synonyms)
'on' acts as a link.
(Other definitions for rails that I've seen before include "Barriers of horizontal bars and supports" , "secretive birds" , "Tracks or barriers" , "Water birds" , "Metal bars of a barrier" .)