Firm taking on new railway guard (6)
I believe the answer is:
sentry
'guard' is the definition.
(I know that sentry is a type of security guard)
'firm taking on new railway' is the wordplay.
'firm' becomes 'set' (both can mean fixed).
'taking on' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'railway' becomes 'ry' (abbreviation for railway).
'set' placed around 'n' is 'sent'.
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "Sentinel, watch" , "Watchperson" , "One posted on guard" , "Military guard" , "Guard on watch" .)