The Ponte Vecchio crosses it
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">the ponte vecchio crosses it</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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?" .)