May be left forbidding (8)

I believe the answer is:

sinister

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

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for sinister that I've seen before include "Threatening evil on the heraldic left" , "Suggestive of evil, being lefthanded" , "Spooky" , "Suggesting or threatening evil or danger" , "'Ominous, baleful (8)'" .)

I've seen this clue in the Sydney Morning Herald.

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