Son, naughty boy, left school a dunce (9)
I believe the answer is:
simpleton
'a dunce' is the definition.
(I've seen this before)
'son naughty boy left school' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'naughty boy' becomes 'imp' (imp can mean a misbehaving child).
'left' becomes 'l' (common abbreviation).
'school' becomes 'eton' (famous UK public school).
's'+'imp'+'l'+'eton'='SIMPLETON'
(Other definitions for simpleton that I've seen before include "Fool" , "Dope" , "Natural" , "Foolish or gullible person" , "Halfwit" .)