Erstwhile plum duffer (5)
I believe the answer is:
prune
This clue is a double definition.
'erstwhile plum' is the first definition.
(a prune is a dried plum)
'duffer' is the second definition.
(both can mean a stupid person)
(Other definitions for prune that I've seen before include "Cut back the dried plum" , "Trim (tree); dried plum" , "Trim - dried fruit" , "A fruit; to cut back" , "Dull person" .)