Not in one's right mind (3,6,6)
I believe the answer is:
non compos mentis
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not in one's right mind</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 non compos mentis that I've seen before include "Not fit to try" , "crackers" , "not quite all there" , "Not in one's right mind" , "mad" .)