Mentally slow, not acute? (6)
I believe the answer is:
obtuse
I believe this is a double definition.
'mentally slow' is the first definition.
(I've seen this before)
'not acute?' is the second definition.
('obtuse' is the opposite to 'acute')
(Other definitions for obtuse that I've seen before include "Blunt, dull-witted" , "angle, perhaps" , "Annoyingly insensitive" , "A stupid angle?" , "Mentally slow - kind of angle" .)