Guard ignoring southern doorway (5)
I believe the answer is:
entry
'doorway' is the definition.
(synonyms)
'guard ignoring southern' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'ignoring' suggests deleting specific letters.
'southern' becomes 's' (abbreviation).
'sentry' with 's' taken away is 'ENTRY'.
(Other definitions for entry that I've seen before include "Lobby" , "List of competitors" , "Gate" , "Competitors - way in" , "all runners" .)