Guard overlooking south gate (5)
I believe the answer is:
entry
'gate' is the definition.
(I've seen this in another clue)
'guard overlooking south' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'overlooking' is a deletion indicator.
'south' becomes 's' (abbreviation).
'sentry' with 's' taken out is 'ENTRY'.
(Other definitions for entry that I've seen before include "The act of going in" , "Competitors - way in" , "Admission" , "List of candidates" , "Admittance" .)