Old, old city belonging to us (3)
I believe the answer is:
our
'belonging to us' is the definition.
(I've seen this before)
'old old city' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'old city' becomes 'ur' (ancient city state).
'o'+'ur'='OUR'
(Other definitions for our that I've seen before include "''Now is the winter of ... discontent'' (Richard III)" , "group possession?" , "Belonging to us" , "Yours and mine" , "Relating to us" .)