Caspian Sea feeder (4)
I believe the answer is:
ural
'caspian sea feeder' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ural that I've seen before include "Mountain range in Russia" , "Russian river giving name to mountain range" , "Russian river flowing into the Caspian Sea" , "Russian river, rising in the mountains between Europe and Asia" , "River feeding Caspian Sea" .)