City on the Arno (4)
I believe the answer is:
'city on the arno' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pisa that I've seen before include "Italian city with leaning tower" , "Site of the famous leaning tower" , "Location of leaning tower" , "City with listed building" , "Italian city with famous tower" .)