How one checked authenticity of old money, then some new (7)

I believe the answer is:

bitcoin

'how one checked authenticity of old money' is the definition.
(people once bit coins to check they were real)

I don't understand the rest of the clue.

(Other definitions for bitcoin that I've seen before include "new money" , "Digital currency" .)

I've seen this clue in The Guardian.

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