Artist carrying can shouts at pack (7)

I believe the answer is:

halloos

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'shouts at pack' is the definition.
'halloos' can be an answer for 'shouts' (hallooing is a kind of shouting). I'm unsure of the 'at pack' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

I've seen this clue in The Guardian.

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