Undermine brat with look (6)
I believe the answer is:
impair
'undermine' is the definition.
(thesaurus)
'brat with look' is the wordplay.
'brat' becomes 'imp'.
'with' is a charade indicator (letters next to each other).
'look' becomes 'air' (I've seen this before).
'imp'+'air'='IMPAIR'
(Other definitions for impair that I've seen before include "Damage or injure" , "Hurt or handicap" , "Weaken, damage" , "Hobble" , "Damage or handicap" .)