Abrupt rudeness upset client (3-4)

I believe the answer is:

end-user

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This clue could be a double definition.

'abrupt rudeness upset' is the first definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'client' is the second definition.
(I've seen this before)

Can you help me to learn more?

I've seen this clue in The Independent.

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