Guard southern passageway (6)
I believe the answer is:
sentry
'guard' is the definition.
(I know that sentry is a type of security guard)
'southern passageway' is the wordplay.
'southern' becomes 's' (abbreviation).
'passageway' becomes 'entry' (I've seen this in another clue).
's'+'entry'='SENTRY'
(Other definitions for sentry that I've seen before include "Soldier on guard" , "One posted on guard" , "Person standing watch" , "Mil. guard" , "Watchperson" .)