Fool with a second moral offence, a murderer (8)
I believe the answer is:
assassin
'a murderer' is the definition.
(thesaurus)
'fool with a second moral offence' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool**).
'with' means one lot of letters go next to another.
'a second' becomes 'as' (I've seen this in other clues).
'moral offence' becomes 'sin' (I've seen this before).
'ass'+'as'+'sin'='ASSASSIN'
(Other definitions for assassin that I've seen before include "Booth, for example" , "A killer of political or religious leader" , "Killer of prominent figure" , "Political/religious killer" , "Carlos the Jackal was one" .)