The Ponte Vecchio crosses it (4)
I believe the answer is:
arno
'the ponte vecchio crosses it' 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 "Florence's river" , "River of 11 across" , "Banker" , "River in Tuscany" , "River of 4 down?" .)