Lacking intelligence (6)
'lacking intelligence' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stupid that I've seen before include "Not intelligent, thick" , "Asinine" , "Not sensible or intelligent" , "dope" , "Barmy" .)
(Other definitions for witless that I've seen before include "Out of one's mind (maybe scared)" , "Lacking sense, understanding or judgment" , "Foolish, stupid" , "Daft" .)