Spoil naughty child's appearance (6)
I believe the answer is:
impair
'spoil' is the definition.
('impair' can be a synonym of 'spoil')
'naughty child's appearance' is the wordplay.
'naughty child' becomes 'imp' (I've seen this before).
'appearance' becomes 'air' (I've seen this before).
'imp'+'air'='IMPAIR'
(Other definitions for impair that I've seen before include "Spoil, degrade" , "blunt" , "Weaken, damage" , "Hobble" , "'Mar, damage (6)'" .)