Guard dispatched to railway? (6)
I believe the answer is:
sentry
'guard' is the definition.
(I know that sentry is a type of guard)
'dispatched to railway?' is the wordplay.
'dispatched' becomes 'sent' (I've seen this in another clue).
'to railway?' becomes 'ry' (abbreviation for railway).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "One who stands guard" , "Guard on watch" , "Mil. guard" , "Picket" , "One posted on guard" .)