Reject northern support for branch line? (5)
I believe the answer is:
spurn
'reject' is the definition.
(I know that reject can be written as spurn)
'northern support for branch line?' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'support for' says to put letters next to each other (in a down clue, some letters 'support' others).
'branch line?' becomes 'spur' ('spur' can be a synonym of 'branch line').
'n' put after 'spur' is 'SPURN'.
(Other definitions for spurn that I've seen before include "Reject with contempt" , "turn away" , "Reject scornfully, say unwanted suitor" , "Despise" , "discard" .)