One of those tradesmen in business jointly? (7)

I believe the answer is:

butcher

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'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

I can't explain the remainder of the clue.

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(Other definitions for butcher that I've seen before include "To hack into pieces" , "Kill brutally" , "In EastEnders, Pat maybe" , "Retailer of meat" , "more macho" .)

I've seen this clue in the Evening Standard.

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