People adding padding help trousers’ alteration (12)

I believe the answer is:

upholsterers

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'people' is the definition.
The answer is an example.

'adding padding help trousers' alteration' is the wordplay.
'help trousers' anagrams to 'UPHOLSTERERS'.
However, I am not clear how this is indicated.

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I've seen this clue in The Guardian.

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