Harm with fairy music (6)
I believe the answer is:
impair
'harm' is the definition.
(I've seen this before)
'fairy music' is the wordplay.
'fairy' becomes 'imp' (imp is a kind of fairy).
'music' becomes 'air' (air is a kind of music).
'imp'+'air'='IMPAIR'
'with' is the link.
(Other definitions for impair that I've seen before include "'Mar, damage (6)'" , "Spoil, degrade" , "Harm" , "Weaken, damage" , "Spoil strength, quality of" .)