Greenery ever changing round many an old city (7)
I believe the answer is:
verdure
'greenery' is the definition.
('verdure' can be a synonym of 'greenery')
'ever changing round many an old city' is the wordplay.
'changing' is an anagram indicator.
'round' means one lot of letters goes inside another.
'many' becomes 'd'.
'an old city' becomes 'ur' (ancient city state).
'ever' anagrammed gives 'vere'.
'd'+'ur'='dur'
'vere' placed around 'dur' is 'VERDURE'.
(Other definitions for verdure that I've seen before include "Greenery, in a French sort of way" , "Flourishing green vegetation" .)