As joined by Jack? (5)

I believe the answer is:

union

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'jack?' is the definition.
(thesaurus)

I don't understand how the remainder of the clue works.

(Other definitions for union that I've seen before include "Workers' organisation or marriage" , "Coupling, merger" , "Equity is one" , "Marriage; trade assoc." , "The coming together of separate parts" .)

I've seen this clue in the Evening Standard.

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