Killing of a king (8)
I believe the answer is:
regicide
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">killing of a king</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 regicide that I've seen before include "doing away with top man" , "King-killer" , "Killing monarch" , "killing of man at the top" , "What Macbeth did" .)