This acid rain alarmed campaigners (9,3)

I believe the answer is:

christian aid

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'alarmed campaigners' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'this acid rain' is the wordplay.
'this acid rain' can be anagrammed to 'CHRISTIAN AID'.
Though, I am unsure how the anagram is indicated.

Can you help me to learn more?

I've seen this clue in The Everyman puzzle in The Observer.

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