Son, mischievous child, revealed dope (9)
I believe the answer is:
simpleton
'dope' is the definition.
(I've seen this before)
'son mischievous child revealed' is the wordplay.
'son' becomes 's'.
'mischievous child' becomes 'imp' (I've seen this before).
'revealed' becomes 'leton' (I've seen this in other clues).
's'+'imp'+'leton'='SIMPLETON'
(Other definitions for simpleton that I've seen before include "Natural" , "Ninny" , "Unintelligent person" , "Halfwit" , "Gullible type" .)