In the middle of the inspection, the promissory notes were found to be utterly implausible. (8)

I believe the answer is:

specious

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'in' is the definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?

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(Other definitions for specious that I've seen before include "Copies us, being superficially good" , "Plausibly deceptive" , "Misleadingly plausible" , "Superficially attractive" , "Deceptively pleasing" .)

I've seen this clue in The Irish Times.

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