7 21 at Dido's place (8)
I believe the answer is:
carthage
'dido's place' is the definition.
(Dido was a Queen of Carthage)
I can't explain the remainder of the clue.
(Other definitions for carthage that I've seen before include "Ancient N. African city" , "Hannibal's home city" , "Dido was queen of this ancient city in N Africa" , "Phoenician city state near modern Tunis" , "It fought the Punic wars against ancient Rome" .)