Europe's second-longest river (6)
I believe the answer is:
danube
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">europe's second-longest river</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 danube that I've seen before include "flower" , "Vienna's river" , "It's blue according to the song" , "Central European river" , "River flowing through Belgrade, Budapest and Vienna" .)