I am above ancient city in China that's undeveloped (8)
I believe the answer is:
immature
'undeveloped' is the definition.
(similar in meaning)
'i am above ancient city in china' is the wordplay.
'i am' becomes 'I'm' (contraction).
'above' says to put letters next to each other.
'ancient city' becomes 'ur' (ancient Sumerian city state).
'in' indicates putting letters inside.
'china' becomes 'mate' (china plate is Cockney rhyming slang for mate).
'ur' inserted inside 'mate' is 'mature'.
'im'+'mature'='IMMATURE'
'that's' acts as a link.
(Other definitions for immature that I've seen before include "Irate mum? (anag)" , "Not yet adult or fully developed" , "Not yet fully-developed" , "Not yet fully grown or developed" , "Not yet adult or fully grown" .)