Dim-witted baronet stuck in river (6)
I believe the answer is:
obtuse
'dim-witted' is the definition.
(thesaurus)
'baronet stuck in river' is the wordplay.
'baronet' becomes 'bt'.
'stuck in' indicates putting letters inside.
'river' becomes 'ouse' (Yorkshire river).
'bt' inserted within 'ouse' is 'OBTUSE'.
(Other definitions for obtuse that I've seen before include "Annoyingly slow to understand" , "A stupid angle?" , "'Dense, slow (6)'" , "Thick, blunt" , "Mentally slow - kind of angle" .)