Eurasian border river (4)
I believe the answer is:
ural
'eurasian border 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 "Europe/Asia river" , "The .... Mountains divide Europe and Asia" , "Russian river flowing into the Caspian Sea" , "The - mountains separate Asia and Europe" , "River feeding Caspian Sea" .)