Guard won't open door (5)
I believe the answer is:
entry
'door' is the definition.
(synonyms)
'guard won't open' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'won't open' means to remove the first letter.
'sentry' with its first letter taken off is 'ENTRY'.
(Other definitions for entry that I've seen before include "Door, access" , "List of candidates" , "vestibule" , "Record - number of contestants" , "Access point" .)