Reason for action (6)
I believe the answer is:
motive
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">reason for action</span>' is the definition.<br/ ><span class="explanation-format__explanation">(what motivates something)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for motive that I've seen before include "Purpose in doing something" , "Reason behind an action" , "Underlying intention" , "Reason for doing something, say murder" , "Rationale, purpose" .)