Originally, a member of a secretive sect in the Middle East, which threatened local caliphates and killed crusaders (8)

I believe the answer is:

assassin

(Other definitions for assassin that I've seen before include "Bravo!" , "One murders for political or monetary reasons" , "Killer of prominent figure" , "Murderer (of prominent figure)" , "Killer of famous person" .)

I've seen this clue in The Sunday Times and The Times.

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