Simpleton of a sort! (3)
I believe the answer is:
oaf
'simpleton' is the definition.
(I've seen this in another clue)
'of a sort' is the wordplay.
'sort' indicates an anagram.
'of'+'a'='ofa'
'ofa' is an anagram of 'OAF'.
(Other definitions for oaf that I've seen before include "Yahoo" , "Lout; dolt" , "Clumsy fool" , "Awkward stupid person" , "A clumsy boor" .)