I have turned left, not right (4)
I believe the answer is:
evil
'not right' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'i have turned left' is the wordplay.
'i have' becomes 'ive' (contraction of 'I have').
'turned' says the letters should be written backwards.
'left' becomes 'L' (common abbreviation).
'ive' in reverse letter order is 'evi'.
'evi'+'l'='EVIL'
Can you help me to learn more?
(Other definitions for evil that I've seen before include "Profoundly malevolent" , "Profoundly wicked" , "Extremely unpleasant" , "Morally reprehensible" , "Wicked, sinful" .)