A very old tsar rebuilt this city (7)
I believe the answer is:
saratov
'this city' is the definition.
(Saratov is an example)
'a very old tsar rebuilt' is the wordplay.
'very' becomes 'v' (common abbreviation).
'old' becomes 'o' (abbreviation).
'rebuilt' indicates anagramming the letters.
'a'+'v'+'o'+'tsar'='avotsar'
'avotsar' is an anagram of 'SARATOV'.
(Other definitions for saratov that I've seen before include "Russian city" , "city on the Volga" .)