Service manual? (6)

I believe the answer is:

hymnal

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'service manual?' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?

This is all the clue.

Can you help me to learn more?

(Other definitions for hymnal that I've seen before include "Book used in church" , "Book of church songs" , "Book of church music" , "Book of religious songs" , "Book of verses expressing 20" .)

I've seen this clue in The Times.

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