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A couple and a threesome ensure no vacancies (4,5)

Ross

I believe the answer is:

full house

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'a couple' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I can't see?

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

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(Other definitions for full house that I've seen before include "Rather good hand" , "well-attended party" , "Promising hand" , "capacity crowd" , "building used a good deal?" .)

I've seen this clue in The Daily Express.
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