Wicked demon decapitated (4)
I believe the answer is:
evil
'wicked' is the definition.
(similar in meaning)
'demon decapitated' is the wordplay.
'demon' becomes 'devil' (synonyms).
'decapitated' means to remove the first letter.
'devil' with its first letter removed is 'EVIL'.
(Other definitions for evil that I've seen before include "Immorality" , "Morally bad" , "'The ... that men do lives after them' (Shakespeare)" , "Profoundly wicked" , "Profoundly immoral" .)