Guard despatched by railway (6)
I believe the answer is:
sentry
'guard' is the definition.
(I know that sentry is a type of security guard)
'despatched by railway' is the wordplay.
'despatched' becomes 'sent' (to despatch is to send off).
'by' means one lot of letters go next to another.
'railway' becomes 'ry' (abbreviation).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "One keeps watch" , "Military lookout" , "Watchperson" , "Lookout person" , "Sentinel, watch" .)