Quiet mass improved with liberal hymns (6)
I believe the answer is:
psalms
'hymns' is the definition.
(psalm is a type of hymn)
'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' mixed up with 'l' can make 'salms'.
'p'+'salms'='PSALMS'
(Other definitions for psalms that I've seen before include "Old Testament book of sacred songs" , "Sacred songs or hymns" , "Sacred songs or poems" , "Religious books" , "Religious songs" .)