The most appropriate time to leave ancient city (6)
I believe the answer is:
thebes
'ancient city' is the definition.
(ancient Greek city)
'the most appropriate time to leave' is the wordplay.
'most appropriate' becomes 'best' (best-suited).
'time' becomes 't' (abbreviation).
'to leave' suggests deleting specific letters.
'best' with 't' taken away is 'bes'.
'the'+'bes'='THEBES'
(Other definitions for thebes that I've seen before include "place in Greece" , "Classical city" , "Old city of Greece or Egypt" , "Greek city founded by Cadmus" , "Greek city of Oedipus" .)