River joining the Tigris (9)
I believe the answer is:
euphrates
'river joining the tigris' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for euphrates that I've seen before include "running water in Iraq" , "River flowing through Turkey, Syria and Iraq" , "Asian flower" , "some running water in Middle East" , "River joining the Tigris" .)