Soldier's opening door for lookout (6)
I believe the answer is:
sentry
'lookout' is the definition.
(synonyms)
'soldier's opening door' is the wordplay.
'opening' suggests taking the first letters.
'door' becomes 'entry' (synonyms).
The initial letter of 'soldiers' is 's'.
's'+'entry'='SENTRY'
'for' is the link.
(Other definitions for sentry that I've seen before include "One posted on guard" , "Watchman" , "Lookout soldier" , "Military guard" , "Person posted to keep watch for some anticipated event" .)