Fail to stress indication of softness in some carpeting? (9)

I believe the answer is:

underplay

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'fail to stress' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for underplay that I've seen before include "having to act with restraint" , "Reduce stress on" , "Perform (a role) in a restrained way" , "Fail to stress" , "show restraint" .)

I've seen this clue in The Times.

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