Guard leaving southern gate (5)
I believe the answer is:
entry
'gate' is the definition.
(I've seen this before)
'guard leaving southern' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'leaving' indicates named letters should be taken away.
'southern' becomes 's' (abbreviation).
'sentry' with 's' taken away is 'ENTRY'.
(Other definitions for entry that I've seen before include "Ingress, portal" , "Debut" , "Doorway" , "Admittance" , "Way in; item in list" .)