One books trio playing religious music (7)

I believe the answer is:

introit

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'religious music' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?

'one books trio playing' is the wordplay.
'one' becomes 'i' (Roman numeral).
'books' becomes 'NT' (abbreviation for New Testament).
'playing' is an anagram indicator.
'trio' anagrammed gives 'roit'.
'i'+'nt'+'roit'='INTROIT'

Can you help me to learn more?

(Other definitions for introit that I've seen before include "chant" , "Pre-Eucharist prayer" , "Type of psalm" , "musical piece" , "Choral piece" .)

I've seen this clue in The Times.

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