Unionists, often married (7)

I believe the answer is:

couples

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

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for couples that I've seen before include "People with their partners" , "Pairs" .)

I've seen this clue in the Evening Standard.

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