Quiet mass improved with liberal songs (6)
I believe the answer is:
psalms
'songs' is the definition.
(I have seen 'Biblical songs ' mean 'psalms' so perhaps 'songs' could also mean 'psalms')
'quiet mass improved with liberal' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'improved with' indicates anagramming the letters.
'liberal' becomes 'l' (abbreviation).
'mass' with its letters anagrammed with 'l' can make 'salms'.
'p'+'salms'='PSALMS'
(Other definitions for psalms that I've seen before include "David's hymns" , "Religious books" , "Biblical songs" , "in the Bible?" , "Religious songs" .)