David's in the services (6)

I believe the answer is:

psalms

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'services' is the definition.
Although both the answer and definition are plural nouns, I don't see how they can define each other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for psalms that I've seen before include "Old Testament book of sacred songs" , "OT book" , "Religious books" , "David's hymns" , "Move fast, noisily" .)

I've seen this clue in The Globe and Mail.

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