Durable conclusion about old city (8)
I believe the answer is:
enduring
'durable' is the definition.
(thesaurus)
'conclusion about old city' is the wordplay.
'conclusion' becomes 'ending' (synonyms).
'about' means one lot of letters goes inside another.
'old city' becomes 'ur' (ancient Sumerian city state).
'ending' placed around 'ur' is 'ENDURING'.
(Other definitions for enduring that I've seen before include "Lasting' tolerating" , "Bearing patiently or lasting" , "Going through" , "Lasting a very long time, despite hardship (8)" , "Lasting, unceasing" .)