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' suggests deleting specific letters.
'southern' becomes 's' (abbreviation).
'sentry' with 's' taken out is 'ENTRY'.
(Other definitions for entry that I've seen before include "Ingress, portal" , "Debut" , "Doorway" , "Admittance" , "Way in; item in list" .)