Son, little rascal, revealed as idiot (9)
I believe the answer is:
simpleton
'idiot' is the definition.
(I've seen this before)
'son little rascal revealed' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'little rascal' becomes 'imp' (imp can mean a misbehaving child).
'revealed' becomes 'leton' (to let on about something is to reveal it).
's'+'imp'+'leton'='SIMPLETON'
'as' acts as a link.
(Other definitions for simpleton that I've seen before include "Fool" , "One very slow" , "Ass" , "Blockhead" , "Simon for one" .)