Wicked monster beheaded (4)
I believe the answer is:
evil
'wicked' is the definition.
('evil' is associated in meaning with 'wicked')
'monster beheaded' is the wordplay.
'monster' becomes 'devil' ('devil' can be a synonym of 'monster').
'beheaded' means to remove the first letter.
'devil' with its first letter taken off is 'EVIL'.
(Other definitions for evil that I've seen before include "Wicked - social problem" , "Vile (anag) -- really bad" , "Live kind of badness" , "Wicked, sinful" , "Morally bad" .)