Europe's second-longest river (6)
I believe the answer is:
danube
'europe's second-longest river' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)