Sound reason to have created bitcoin? (5)

I believe the answer is:

mined

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'sound' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?

I can't explain the rest of the clue.

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(Other definitions for mined that I've seen before include "Quarried" , "Extracted minerals from the earth" , "Booby-trapped; dug ores from" , "Dug from pit" , "Brought up" .)

I've seen this clue in the Sydney Morning Herald.

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