Chant used in Holy Communion services (5,3)

I believe the answer is:

agnus dei

(Other definitions for agnus dei that I've seen before include "Christian emblem" , "part in requiem" , "Latin liturgical prayer, Lamb of God" , "'Latin mass prayer, Lamb of God (5,3)'" , "sacrificial offering" .)

I've seen this clue in The Telegraph.

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