Song priest sang at Latin Mass, originally (5)
I believe the answer is:
psalm
'song' is the definition.
(I've seen this before)
'priest sang at latin mass originally' is the wordplay.
'priest' becomes 'p' (ecclesiastical abbreviation).
'originally' suggests taking the first letters.
The initial letters of 'sang at latin mass' is 'salm'.
'p'+'salm'='PSALM'
(Other definitions for psalm that I've seen before include "Old Testament lyrical poem or prayer" , "Religious song" , "Devotional song/hymn" , "Devotional hymn or song" , "OT hymn" .)