One of those legal smoothies (4)

I believe the answer is:

silk

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for silk that I've seen before include "Smooth material" , "Fibre produced by worms" , "Glossy fibre" , "Fabric from larvae" , "it's taken by QC" .)

I've seen this clue in the Evening Standard.

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