Guard dispatched by railway (6)
I believe the answer is:
sentry
'guard' is the definition.
(sentry is a kind of guard)
'dispatched by railway' is the wordplay.
'dispatched' becomes 'sent' (I've seen this before).
'by' means one lot of letters go next to another.
'railway' becomes 'ry' (abbreviation for railway).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "Watchman" , "One keeps guard" , "Lookout person" , "Sentinel, watch" , "Sentinel, look-out" .)