Unpremeditated (11)
I believe the answer is:
spontaneous
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unpremeditated</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for spontaneous that I've seen before include "Impromptu" , "Unprompted" , "Cause of Mr Krook's death" , "Impulsive, unpremeditated" .)