Dozy bachelor finally lost in river (6)
I believe the answer is:
obtuse
'dozy' is the definition.
(both can mean stupid)
'bachelor finally lost in river' is the wordplay.
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'finally' suggests the final letters.
'in' is an insertion indicator.
'river' becomes 'ouse' (Yorkshire river).
The last letter of 'lost' is 't'.
'b'+'t'='bt'
'bt' put within 'ouse' is 'OBTUSE'.
(Other definitions for obtuse that I've seen before include "Annoyingly insensitive" , "Dense, dim about the angle" , "Can't quite see the angle" , "Blunt or unintelligent" , "Not very perceptive or sensitive" .)