Stupid baronet plunged into river (6)
I believe the answer is:
obtuse
'stupid' is the definition.
(similar in meaning)
'baronet plunged into river' is the wordplay.
'baronet' becomes 'bt'.
'plunged into' means one lot of letters goes inside another.
'river' becomes 'ouse' (Yorkshire river).
'bt' placed inside 'ouse' is 'OBTUSE'.
(Other definitions for obtuse that I've seen before include "(Of angle) between 90° and 180° - dull" , "To be thus blunt" , "Dim-witted" , "Dull, non-perceptive" , "Blunt - stupid - insensitive" .)