Group adds individual piece to religious text (5)
I believe the answer is:
nonet
'group' is the definition.
(group of nine musicians)
'adds individual piece to religious text' is the wordplay.
I cannot quite understand how this works, but
'individual' could be 'one' (an individual is one person) and 'one' is located in the answer.
'piece' could be 'n' (abbreviation for Knight in chess notation) and 'n' is found in the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nonet that I've seen before include "Tonne (anag.); music for a group" , "Piece of music for nine" , "Multi-figure composition" , "baseball team" , "Musical work for a set number of performers" .)