Don't you believe it! (7)

I believe the answer is:

fiction

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'it' is the definition.
The definition suggests a singular noun which matches the answer.

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

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(Other definitions for fiction that I've seen before include "Fabricated story" , "Something invented" , "Novel writing" , "Literary genre" , "Imaginary prose" .)

I've seen this clue in the Evening Standard.

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