Ancient city almost unrivalled (6)

I believe the answer is:

thebes

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'ancient city' is the definition.
(ancient Greek city)

I don't understand the remainder of the clue.

(Other definitions for thebes that I've seen before include "Seven Against - (Aeschylus)" , "Ancient Egyptian or Greek city" , "Greek city founded by Cadmus" , "Old Egyptian city" , "Old city of Greece or Egypt" .)

I've seen this clue in The Times.

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