In it he besieged ancient Greek city (6)

I believe the answer is:

thebes

'greek city' is the definition.
(Thebes is a Greek city)

'in it he besieged ancient' is the wordplay.
'in' indicates the answer is hidden within the clue.
'THEBES' is hidden within 'it he besieged ancient'.

(Other definitions for thebes that I've seen before include "Greek city of Oedipus" , "Classical city" , "Ancient city, a rival to Athens" , "Old Egyptian city" , "Greek city founded by Cadmus" .)

I've seen this clue in The Telegraph.

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