European border river (4)
I believe the answer is:
oder
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">european border 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 oder that I've seen before include "waterway" , "East European river" , "flowing water" , "Rode around the European river" , "River on border of Poland and Germany" .)