No doubt it was used in bygone days (8)

I believe the answer is:

forsooth

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'no' is the definition.
Both the answer and definition are adverbs. Perhaps they are linked in a way I don't understand?

I don't understand how the remainder of the clue works.

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(Other definitions for forsooth that I've seen before include "Indeed (archaic)" , "Marry" , "In truth (arch)" , "certainly" , "truly" .)

I've seen this clue in The Globe and Mail.

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