Weight in carats, perhaps? (5)

I believe the answer is:

stone

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'weight' is the definition.
(stone is a unit of weight)

I can't explain the rest of the clue.

(Other definitions for stone that I've seen before include "Rock; hard seed" , "See 1" , "Endocarp" , "Rock; cherry seed" , "Kernel" .)

I've seen this clue in the Evening Standard.

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