Pisa's river (4)
I believe the answer is:
arno
'pisa's river' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arno that I've seen before include "Pisa's river" , "River flowing through Florence and Pisa" , "European river" , "25's river" , "River of 11 across" .)