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' ('ending' can be a synonym of 'conclusion').
'about' indicates putting letters inside.
'old city' becomes 'ur' (ancient city state).
'ending' enclosing '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" .)