Being left is rather frightening (8)
I believe the answer is:
sinister
'being left' is the definition.
(I've seen this in another clue)
'is rather frightening' is the wordplay.
I cannot really see how this works, but
'is' is found in the answer.
The remaining letters 'sinter' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sinister that I've seen before include ", nun going round home" , "Resist in way that is creepy" , "Threatening the left in heraldry" , "On the left (of shield) (her.); wicked, criminal" , "Suggestive of evil, being lefthanded" .)