River of Russia (4)
I believe the answer is:
ural
'river of russia' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ural that I've seen before include "The - mountains separate Asia and Europe" , "Russian river flowing into the Caspian Sea" , "Mountain range between Europe and Asia" , "Europe/Asia river" , "2,400 km river flowing through Russia and Kazakhstan into the Caspian Sea" .)