Dispatched with railway guard (6)
I believe the answer is:
sentry
'guard' is the definition.
(I know that sentry is a type of security guard)
'dispatched with railway' is the wordplay.
'dispatched' becomes 'sent' (I've seen this in another clue).
'with' is a charade indicator (letters next to each other).
'railway' becomes 'ry' (abbreviation).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "Sentinel, look-out" , "Lookout person" , "Guard on watch" , "Mil. guard" , "Lookout soldier" .)