It's cut not paste (7)

I believe the answer is:

diamond

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'paste' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for diamond that I've seen before include "60th anniversary" , "Gemstone - baseball field" , "suited one" , "Kohinoor, Hope" , "Form of carbon" .)

I've seen this clue in The Independent.

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