Sacred song, second to be introduced to mate before mass (5)
I believe the answer is:
psalm
'sacred song' is the definition.
(I've seen this before)
'second to be introduced to mate before mass' is the wordplay.
'second' becomes 's' (common abbreviation).
'to be introduced to' is an insertion indicator.
'mate' becomes 'pal' (I've seen this before**).
'before' says to put letters next to each other.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
's' placed into 'pal' is 'psal'.
'psal'+'m'='PSALM'
(Other definitions for psalm that I've seen before include "Song of David" , "OT hymn" , "Lamps (anag) -- song" , "Lamps for a sacred song" , "Sacred song of praise" .)