Fertile Crescent river (6)
I believe the answer is:
tigris
'fertile crescent river' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tigris that I've seen before include "Great river of 30 across" , "It flows from Turkey to the Persian Gulf" , "River of Baghdad" , "Great river of Mesopotamia" , "Euphrates companion river" .)