Sacred hymn first performed solo at Lenten mass (5)
I believe the answer is:
psalm
'sacred hymn' is the definition.
(I've seen this in another clue)
'first performed solo at lenten mass' is the wordplay.
'first' indicates taking the first letters.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
The initial letters of 'performed solo at lenten' is 'psal'.
'psal'+'m'='PSALM'
(Other definitions for psalm that I've seen before include "Sacred poem (written by King David?)" , "Lamps for a sacred song" , "Devotional song/hymn" , "Lamps (anag) -- song" , "Palms (anag.)" .)