Undoing of a dolt! (3)
I believe the answer is:
oaf
'dolt' is the definition.
(I've seen this before)
'undoing of a' is the wordplay.
'undoing' indicates an anagram.
'of'+'a'='ofa'
'ofa' with letters rearranged gives 'OAF'.
(Other definitions for oaf that I've seen before include "Clod" , "Yahoo" , "Clumsy fool" , "buffoon" , "A clumsy boor" .)