Whodunit plot element (6)
I believe the answer is:
motive
'whodunit plot element' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)