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Quack keeping two ducks closer? (4)

Ross

I believe the answer is:

door

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

'quack' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'keeping two ducks closer?' is the wordplay.
'keeping' is an insertion indicator (some letters kept inside).
'two ducks' becomes 'oo'.
'closer?' becomes 'dr' (I can't justify this - if you can you should believe this answer much more).
'oo' inserted inside 'dr' is 'DOOR'.

Can you help me to learn more?

(Other definitions for door that I've seen before include "Swinging barrier" , "Opener" , "Entry point" , "Threshold" , "Alternative to knocker" .)

I've seen this clue in The Times.
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