Guard posted overseeing railway (6)
I believe the answer is:
sentry
'guard' is the definition.
(sentry is a kind of guard)
'posted overseeing railway' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'overseeing' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'railway' becomes 'ry' (abbreviation for railway).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "Look-out, guard" , "Watchman" , "Guard on watch" , "Watchperson" , "Soldier posted to keep watch" .)