Cain was guilty of it though unusually fair and direct (10)
I believe the answer is:
fratricide
'cain was guilty of it' is the definition.
(Cain killed his brother Abel in the Bible)
'unusually fair and direct' is the wordplay.
'unusually' indicates anagramming the letters.
'and' says to put letters next to each other (I've seen this in other clues).
'fair' after 'direct' is 'directfair'.
'directfair' anagrammed gives 'FRATRICIDE'.
'though' acts as a link.
(Other definitions for fratricide that I've seen before include "Murder of a brother" , "Cain's crime" , "The killing of one's brother" .)