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