Guard; lookout (6)
I believe the answer is:
sentry
This clue is a double definition.
'guard' is the first definition.
(I know that sentry is a type of security guard)
'lookout' is the second definition.
(I know that lookout can be written as sentry)
(Other definitions for sentry that I've seen before include "One keeps watch" , "Sentinel, watch" , "Look-out, guard" , "Military guard" , "Person posted to keep watch for some anticipated event" .)