Europe's third-longest river (4)
I believe the answer is:
ural
'europe's third-longest river' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ural that I've seen before include "The - mountains separate Asia and Europe" , "Mountain range in Russia" , "Russian river flowing into the Caspian Sea" , "Russian river giving name to mountain range" , "River of central Russia" .)