Undoing of a lout! (3)
I believe the answer is:
oaf
'lout' is the definition.
(I know that lout can be written as oaf)
'undoing of a' is the wordplay.
'undoing' is an anagram indicator.
'of'+'a'='ofa'
'ofa' is an anagram of 'OAF'.
(Other definitions for oaf that I've seen before include "Barbarian" , "Oik" , "Stupid lout" , "Lout; dolt" , "Lout, lummox" .)