Left being unpleasant (8)
I believe the answer is:
sinister
I believe this clue is a double definition.
'left' is the first definition.
(I've seen this in another clue)
'unpleasant' is the second definition.
(thesaurus)
'being' is the link.
(Other definitions for sinister that I've seen before include "Resist in way that is creepy" , "Malign, ominous" , "Very unpleasant" , "can be injurious" , "Suggestive of evil, threatening harm" .)