A swinger making an entrance (4)

I believe the answer is:

door

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

'entrance' is the definition.
(I know that door is a type of entrance)

I don't understand the remainder of the clue.

(Other definitions for door that I've seen before include "Alternative to knocker" , "Portal" , "See 1" , "Jim Morrison?" , "Threshold" .)

I've seen this clue in the Evening Standard.

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