Open with a shout at a pep rally for a religious song (10)

I believe the answer is:

plainchant

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'religious song' is the definition.
(plainchant is a kind of religious song)

I cannot understand how the remainder of the clue works.

(Other definitions for plainchant that I've seen before include "unison singing" , "form of sacred music" .)

I've seen this clue in The Hindu.

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