Spoil mischievous child with tune (6)
I believe the answer is:
impair
'spoil' is the definition.
(synonyms)
'mischievous child with tune' is the wordplay.
'mischievous child' becomes 'imp' (I've seen this before).
'with' means one lot of letters go next to another.
'tune' becomes 'air' (air can mean a piece of music).
'imp'+'air'='IMPAIR'
(Other definitions for impair that I've seen before include "blunt" , "'Mar, damage (6)'" , "Damage or injure" , "Hobble" , "Cause weakening of, damage" .)