Seat getting lower at one end according to the notes (4)

I believe the answer is:

sofa

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'seat' is the definition.
(I know that sofa is a type of seat)

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

(Other definitions for sofa that I've seen before include "Settee" , "Somewhere to sit" , "Lounge" , "Living room seat" , "Comfy piece of furniture" .)

I've seen this clue in The Telegraph.

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