Well-built railway linked to British Unionist (5)
I believe the answer is:
burly
'well-built' is the definition.
(thesaurus)
'railway linked to british unionist' is the wordplay.
'railway' becomes 'rly'.
'linked to' is a charade indicator (letters next to each other).
'british' becomes 'b' (abbreviation e.g. in BBC).
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'b'+'u'='bu'
'rly' after 'bu' is 'BURLY'.
(Other definitions for burly that I've seen before include "Muscular; strapping" , "Big and thickset" , "Stocky" , "Big and sturdy" , "Muscular and heavily built" .)