Guard small door ... (6)
I believe the answer is:
sentry
'guard' is the definition.
(sentry is a kind of guard)
'small door' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'door' becomes 'entry' ('entry' can be a synonym of 'door').
's'+'entry'='SENTRY'
(Other definitions for sentry that I've seen before include "Military guard" , "Person posted to keep watch for some anticipated event" , "Picket" , "One posted on guard" , "Person standing watch" .)