Cockney pet mad about city (9)
I believe the answer is:
amsterdam
'city' is the definition.
(Amsterdam is an example)
'cockney pet mad about' is the wordplay.
'cockney pet' becomes 'amster' ('hamster' - I've seen this before - said with a cockney accent, dropping the 'H').
'about' is a reversal indicator.
'mad' written backwards gives 'dam'.
'amster'+'dam'='AMSTERDAM'
(Other definitions for amsterdam that I've seen before include "Dutch capital" , "City known for canals" , "Dutch city of canals" , "place of canals" , "city with many canals" .)