Short railway line (6)
I believe the answer is:
siding
'short railway line' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for siding that I've seen before include "Short stretch of railway track off the main line" , "Short piece of railway track used for shunting" , "Railway spur" , "Taking one part in an argument on the spare railway track" , "Section of track" .)