May be left forbidding (8)
I believe the answer is:
'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)'" .)