The most suitable time to depart ancient city (6)
I believe the answer is:
thebes
'ancient city' is the definition.
(ancient Greek city)
'the most suitable time to depart' is the wordplay.
'most suitable' becomes 'best' (I've seen this in another clue).
'time' becomes 't' (abbreviation).
'to depart' suggests deleting specific letters.
'best' with 't' taken out is 'bes'.
'the'+'bes'='THEBES'
(Other definitions for thebes that I've seen before include "Ancient city of upper Egypt" , "Ancient Egyptian city sited at modern-day Luxor" , "Greek city founded by Cadmus" , "Classical city" , "Ancient Egyptian or Greek city" .)