In harmony, all are singing from the same one (6)

I believe the answer is:

hymnal

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'singing from the same one' is the definition.
'hymnal' can be an answer for 'singing' (thesaurus). I am unsure of the 'from the same one' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for hymnal that I've seen before include "Book of religious songs" , "Religious songbook" , "Book of church songs" , "Book used in church" , "Book of sacred songs" .)

I've seen this clue in the Evening Standard.

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