Murderer is a fool because wrong (8)
I believe the answer is:
assassin
'murderer' is the definition.
(I've seen this before)
'fool because wrong' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'because' becomes 'as' (I've seen this in other clues).
'wrong' becomes 'sin' (wrong can mean a sin or crime).
'ass'+'as'+'sin'='ASSASSIN'
'is a' acts as a link.
(Other definitions for assassin that I've seen before include "Hitman" , "Political/religious killer" , "Professional, often political, killer" , "Booth, for example" , "Killer of prominent person" .)