Despatched railway guard (6)
I believe the answer is:
sentry
'guard' is the definition.
(sentry is a kind of guard)
'despatched railway' is the wordplay.
'despatched' becomes 'sent' (I've seen this before).
'railway' becomes 'ry' (abbreviation for railway).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "Soldier on guard" , "Look-out, guard" , "Person standing watch" , "Military lookout" , "Military guard" .)