Look after the naughty child and spoil (6)
I believe the answer is:
impair
'spoil' is the definition.
(I know that spoil can be written as impair)
'look after the naughty child' is the wordplay.
'look' becomes 'air' (something might have a certain air or look).
'after' means one lot of letters go next to another.
'the naughty child' becomes 'imp' (informal term for a misbehaving child).
'air' after 'imp' is 'IMPAIR'.
'and' acts as a link.
(Other definitions for impair that I've seen before include "Damage or injure" , "Spoil, degrade" , "Reduce" , "blunt" , "Hurt or handicap" .)