Crossword Genius

Witch hunt based on lie under oath: say that's not right (5)

Ross

I believe the answer is:

purge

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'witch hunt' is the definition.

I don't understand the rest of the clue.

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(Other definitions for purge that I've seen before include "Purify from sin or guilt" , "empty bowels" , "Removal by cleansing" , "elimination of opponents" , "Remove by cleansing" .)

I've seen this clue in The Guardian.
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