Spain's longest river (4)
I believe the answer is:
ebro
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">spain's longest river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ebro that I've seen before include "European runner" , "River of NE Spain; bore (anag.)" , "Spanish runner" , "Border Hannibal crossed" , "flower" .)