Rail in bar set to break (8)
I believe the answer is:
banister
'rail' is the definition.
(banister is a kind of rail)
'in bar set to break' is the wordplay.
'to break' indicates anagramming the letters.
'in'+'bar'+'set'='inbarset'
'inbarset' anagrammed gives 'BANISTER'.
(Other definitions for banister that I've seen before include "Upright supporting handrail of stairs" , "support at hand on flight" , "Staircase handrail" , "Staircase railing" , "Stair rail" .)