Spoil naughty child with piece of music (6)
I believe the answer is:
impair
'spoil' is the definition.
(I know that spoil can be written as impair)
'naughty child with piece of music' is the wordplay.
'naughty child' becomes 'imp' (informal term for a misbehaving child).
'with' says to put letters next to each other.
'piece of music' becomes 'air' (I've seen this before).
'imp'+'air'='IMPAIR'
(Other definitions for impair that I've seen before include "Make worse or less effective" , "Spoil, degrade" , "Hinder" , "Cause weakening of, damage" , "Hobble" .)