I have turned left, being sinister (4)
I believe the answer is:
evil
'sinister' is the definition.
(similar in meaning)
'i have turned left' is the wordplay.
'i have' becomes 'ive' (contraction of 'I have').
'turned' is a reversal indicator.
'left' becomes 'l' (common abbreviation).
'ive' written backwards gives 'evi'.
'evi'+'l'='EVIL'
'being' acts as a link.
(Other definitions for evil that I've seen before include "Wicked - social problem" , "Villainy" , "Profoundly wicked" , "heinous" , "Malign" .)