Sentry's appearance, old-fashioned (7)
I believe the answer is:
lookout
'sentry's' is the definition.
(synonyms)
'appearance old-fashioned' is the wordplay.
'appearance' becomes 'look' (look is a kind of appearance).
'old-fashioned' becomes 'out' (as opposed to 'in').
'look'+'out'='LOOKOUT'
(Other definitions for lookout that I've seen before include "Person in the crow's nest" , "Beware - keep a sharp eye open" , "View" , "Nightwatchman" , "Observation point" .)