Crossword Genius

This Jack has three crosses to bear (5)

Ross

I believe the answer is:

union

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'this jack' is the definition.
(thesaurus)

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

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(Other definitions for union that I've seen before include "Conjunction" , "Workers' association - marriage" , "Agreement -- wedlock" , "Marriage; trade assoc." , "Togetherness" .)

I've seen this clue in The Globe and Mail.
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