Border river (3,6)
I believe the answer is:
rio grande
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">border river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for rio grande that I've seen before include "Great river bordering USA and Mexico" , "running water" , "River boundary between the US and Mexico" , "Long river" , "Colorado-to-Mexico river" .)