Branch of the railway taking up arms (6)
I believe the answer is:
siding
This clue could be a double definition.
'branch of the railway' is the first definition.
'siding' can be an answer for 'railway' (siding is a kind of railway). I'm unsure of the 'branch of the' bit.
'taking up arms' is the second definition.
'siding' can be an answer for 'taking' (siding is a kind of taking). I am not sure about the 'up arms' bit.
Can you help me to learn more?
(Other definitions for siding that I've seen before include "Section of track" , "Railway spur" , "Short railway branch" , "Taking one part in an argument on the spare railway track" , "Where coaches may be" .)