Guard small door ... (6)
I believe the answer is:
Here is my best explanation:
'guard' is the definition.
(sentry is a kind of guard)
'small door' is the subsidiary indication.
'small' becomes 's' (abbreviation - e.g. clothes size).
'door' becomes 'entry' ('entry' can be a synonym of 'door').
(Other definitions for sentry that I've seen before include "One posted on guard", "Sentinel, look-out", "Mil. guard", "Person standing watch", "Person posted to keep watch for some anticipated event", "Watchperson", "Watch", "Military guard", "Sentinel, watch" and "One who stands guard".)