Underground to spoil new city (10)
I believe the answer is:
metropolis
'city' is the definition.
(I know that Metropolis is a ancient city)
'underground to spoil new' is the wordplay.
'underground' becomes 'metro' (synonyms).
'to' is a charade indicator (letters next to each other).
'new' indicates anagramming the letters.
'spoil' is an anagram of 'polis'.
'metro'+'polis'='METROPOLIS'
(Other definitions for metropolis that I've seen before include "population centre" , "Expressionist movie" , "Very large city" , "Major city of region" , "Main city" .)