Guard small passage (6)
I believe the answer is:
sentry
'guard' is the definition.
(sentry is a kind of guard)
'small passage' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'passage' becomes 'entry'.
's'+'entry'='SENTRY'
(Other definitions for sentry that I've seen before include "Person standing watch" , "Military guard" , "Look-out, guard" , "Sentinel, look-out" , "Watchman" .)