Feles magna agrestis virgata (sed si mavis, flumen in Mesopotamia) (6)

I believe the answer is:

tigris

(Other definitions for tigris that I've seen before include "Grit is found in great river of Iraq" , "It flows from Turkey to the Persian Gulf" , "Great river of 30 across" , "Baghdad's river" , "Euphrates companion river" .)

I've seen this clue in The Times.

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