Magnificence in garden cultivated by old city (8)
I believe the answer is:
grandeur
'magnificence' is the definition.
('grandeur' can be a synonym of 'magnificence')
'garden cultivated by old city' is the wordplay.
'cultivated' is an anagram indicator.
'by' means one lot of letters go next to another.
'old city' becomes 'ur' (ancient city state).
'garden' is an anagram of 'grande'.
'grande'+'ur'='GRANDEUR'
'in' is the link.
(Other definitions for grandeur that I've seen before include "Pomp" , "Impressiveness of appearance" , "Majesty - eminence" , "Loftiness" , "Splendour, impressiveness" .)