One of the two main branches of Islam (4)
I believe the answer is:
shia
'one of the two main branches of islam' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for shia that I've seen before include "A major denomination of Islam" , "A_main branch of Islam" , "One of the two main branches of orthodox Islam" , "A main branch of Islam" .)