Not in one's right mind (3,6,6)
I believe the answer is:
non compos mentis
'not in one's right mind' is the definition.
(I've seen this before)
This is all the clue.
(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" .)