Unlovely boy good for church music (9)
I believe the answer is:
plainsong
'church music' is the definition.
(plainsong is a kind of church music)
'unlovely boy good' is the wordplay.
'unlovely' becomes 'plain'.
'boy' becomes 'son' ('son' can be a synonym of 'boy').
'good' becomes 'g' (abbreviation).
'plain'+'son'+'g'='PLAINSONG'
'for' acts as a link.
(Other definitions for plainsong that I've seen before include "Lied, perhaps" , "Mediaeval church music" , "accompanied vocal music? No" , "Simple melody" , "church melody" .)