I nearly forgot alternate parts of early Mass hymns (6)
I believe the answer is:
psalms
'hymns' is the definition.
(psalm is a type of hymn)
'i nearly forgot alternate parts of early mass' is the wordplay.
'i nearly forgot' becomes 'PS' (as in a PS in a letter).
'alternate parts of' indicates alternate letters.
'early'+'mass'='earlymass'
The alternating letters of 'earlymass' are 'alms'.
'ps'+'alms'='PSALMS'
(Other definitions for psalms that I've seen before include "Devotional songs" , "Religious books" , "They're heard in church" , "Holy songs" , "Sacred songs of praise" .)