River at Orsk (4)
I believe the answer is:
ural
'river at orsk' 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 "Mountain range between Europe and Asia" , "Russian river, rising in the mountains between Europe and Asia" , "Mountains in Russia" , "Russian river flowing into the Caspian Sea" , "The - mountains separate Asia and Europe" .)