Carpeted, maybe, but you can't tread on it (5)

I believe the answer is:

riser

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'tread on it' is the definition.
'riser' can be an answer for 'tread' (thesaurus). I am not sure about the 'on it' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for riser that I've seen before include "one of a number in flight" , "Vertical upward pipe" , "One getting up" , "Part of flight" , "One on the way up" .)

I've seen this clue in the Evening Standard.

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