Small brat revealed to be fool (9)
I believe the answer is:
simpleton
'be fool' is the definition.
(thesaurus)
'small brat revealed' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'brat' becomes 'imp'.
'revealed' becomes 'leton' (I've seen this in other clues).
's'+'imp'+'leton'='SIMPLETON'
'to' acts as a link.
(Other definitions for simpleton that I've seen before include "Ninny" , "Simon for one" , "Ass" , "Idiot" , "Dope" .)