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