A couple and a threesome ensure no vacancies (4,5)
I believe the answer is:
full house
'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.
Can you help me to learn more?
(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?" .)