Whodunit plot element (6)
I believe the answer is:
motive
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">whodunit plot element</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for motive that I've seen before include "Reason for doing something, perhaps a crime" , "Underlying purpose" , "Reason (for acting)" , "Reason behind an action" , "Rationale, purpose" .)