Go down to a bathroom feature (4)

I believe the answer is:

sink

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'go down' is the definition.
(I know that sink is a more specific form of go down)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sink that I've seen before include "Basin for washing up etc" , "Washbasin" , "Pocket (billiards)" , "Descend to a lower level" , "Dip" .)

I've seen this clue in The Mirror.

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