Crossword Genius

They are bound to belong to the same union (3,3,4)

Ross

I believe the answer is:

man and wife

I'm a little stuck... Click here to teach me more about this clue!

'union' is the definition.
The definition and answer can be both groups as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

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

Can you help me to learn more?

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