He's active in the masses observed by the ministry, sometimes getting cross (5,3)

I believe the answer is:

altar boy

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'he's active in the masses' is the definition.
(person taking part in a religious mass)

I don't understand the rest of the clue.

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(Other definitions for altar boy that I've seen before include "Priest's attendant" , "Priest's young assistant during a service" , "Youth who assists priest at mass" , "Young singer" .)

I've seen this clue in The Telegraph.

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