Crossword Genius

Swingers, but there’s something square about them (5)

Ross

I believe the answer is:

doors

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

'swingers' is the definition.
Although both the answer and definition are plural nouns, I can't understand how one could define the other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for doors that I've seen before include "Ways in and out" , "Swinging or sliding barriers" , "Barriers to entrances" , "Openings" , "Portals" .)

I've seen this clue in the Evening Standard.
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