Be turning wicked (4)
I believe the answer is:
evil
'wicked' is the definition.
('evil' is associated in meaning with 'wicked')
'be turning' is the wordplay.
'be' becomes 'live' (living is a kind of being).
'turning' shows that the letters should be reversed in order.
'live' back-to-front is 'EVIL'.
(Other definitions for evil that I've seen before include "'The ... that men do lives after them' (Shakespeare)" , "Satanic" , "Vile (anag) -- really bad" , "Very wicked or bad" , "Depravity, wickedness" .)