Cain's crime (10)
I believe the answer is:
fratricide
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cain's crime</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for fratricide that I've seen before include "The killing of one's brother" , "Cain's crime" , "Murder of a brother" .)