Forbidding some assassin is terrifying (8)

I believe the answer is:

sinister

'forbidding' is the definition.
(I know that forbidding can be written as sinister)

'some assassin is terrifying' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'SINISTER' can be found hidden inside 'assassin is terrifying'.

(Other definitions for sinister that I've seen before include "Menacing" , "'Ominous, baleful (8)'" , "Disquieting" , "Threatening the left in heraldry" , "not right!" .)

I've seen this clue in The Telegraph.

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