Dispatched railway guard (6)
I believe the answer is:
sentry
'guard' is the definition.
(I know that sentry is a type of security guard)
'dispatched railway' is the wordplay.
'dispatched' becomes 'sent' (I've seen this in another clue).
'railway' becomes 'ry' (abbreviation).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "Watchperson" , "Watchman" , "Person standing watch" , "Military guard" , "One posted on guard" .)