Crossword Genius

Quack to keep ducks closer? (4)

Ross

I believe the answer is:

door

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'quack' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'to keep ducks closer?' is the wordplay.
'to keep' indicates putting letters inside.
'ducks' becomes 'OO' (two O's - duck means zero in cricket).
'closer?' becomes 'dr' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'oo' going into 'dr' is 'DOOR'.

Can you help me to learn more?

(Other definitions for door that I've seen before include "Entrance barrier" , "See 1" , "Moveable structure at an entrance" , "Portal" , "Opener" .)

I've seen this clue in The Sun.
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