Armchairs should be this material (5)

I believe the answer is:

satin

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'this material' is the definition.
(satin is a kind of material)

I cannot understand how the rest of the clue works.

(Other definitions for satin that I've seen before include "Material with glossy front and dull back" , "Stain on shiny cloth" , "Fine material" , "Smooth glossy fabric" , "Stain on shiny fabric" .)

I've seen this clue in The Independent.

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