Turned against, being taken in by fib - heinous (4)
I believe the answer is:
evil
'heinous' is the definition.
(I've seen this before)
'turned against being taken in by fib' is the wordplay.
'turned' says the letters should be written in reverse.
'against' becomes 'v' (abbreviation for 'versus').
'being taken in by' is an insertion indicator.
'fib' becomes 'lie' (I've seen this before).
'v' placed within 'lie' is 'live'.
'live' back-to-front is 'EVIL'.
(Other definitions for evil that I've seen before include "Nefarious" , "Morally corrupt" , "Excellent" , "Morally bad" , "Morally wrong" .)