Bars train supporters (5)
I believe the answer is:
rails
I believe this clue is a double definition.
'bars' is the first definition.
(rail is a kind of bar)
'train supporters' is the second definition.
(I've seen this in another clue)
(Other definitions for rails that I've seen before include "Metal bars of a barrier" , "Tracks or barriers" , "secretive birds" , "Water birds" , "Wading birds - dysfunctional person has come off them" .)