Dispatched by railway guard (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' says to put letters next to each other.
'railway' becomes 'ry' (abbreviation for railway).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "Lookout soldier" , "Picket" , "One posted on guard" , "Guard on watch" , "Watchperson" .)