Posted extremely rusty lookout (6)
I believe the answer is:
sentry
'lookout' is the definition.
(I know that lookout can be written as sentry)
'posted extremely rusty' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'extremely' suggests removing the centre (only the extremes of the word are used).
'rusty' with its centre removed is 'ry'.
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "One keeps guard" , "Person standing watch" , "Military guard" , "Watchman" , "Look-out, guard" .)