Support a railway subsidiary (9)
I believe the answer is:
secondary
'subsidiary' is the definition.
('secondary' can be similar in meaning to 'subsidiary')
'support a railway' is the wordplay.
'support' becomes 'second' (I've seen this before**).
'railway' becomes 'ry' (abbreviation for railway).
'second'+'a'+'ry'='SECONDARY'
(Other definitions for secondary that I've seen before include "Subsidiary" , "Relating to education from eleven to eighteen" , "One grade below the first" , "less important" , "Derivative - of minor importance" .)