Big lie about river flowing backwards; a joke

I believe the answer is:

one-liner

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(Other definitions for one-liner that I've seen before include "Witty remark consisting of a short sentence" , "Quick joke?" , "Witty remark in single short sentence" , "Witty remark contained in a single sentence" , "Pithy joke" .)

I've seen this clue in The Telegraph.

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