Stupid baronet plunged in river (6)
I believe the answer is:
obtuse
'stupid' is the definition.
(similar in meaning)
'baronet plunged in river' is the wordplay.
'baronet' becomes 'bt'.
'plunged in' means one lot of letters goes inside another (I've seen this in other clues).
'river' becomes 'ouse' (Yorkshire river).
'bt' inserted within 'ouse' is 'OBTUSE'.
(Other definitions for obtuse that I've seen before include "Thick, blunt" , "Not very bright" , "Mentally slow - kind of angle" , "Dim-witted" , "Orchestra; stripe" .)