Fully developed ancient city in China (6)
I believe the answer is:
mature
'fully developed' is the definition.
(I've seen this before)
'ancient city in china' is the wordplay.
'ancient city' becomes 'ur' (ancient Sumerian city state).
'in' means one lot of letters goes inside another.
'china' becomes 'mate' (china plate is Cockney rhyming slang for mate).
'ur' going within 'mate' is 'MATURE'.
(Other definitions for mature that I've seen before include "Fully grown and developed (6)" , "Ripe or fully grown" , "Old film star" , "due to be paid" , "reach the end of term" .)