Not standing up for the truth? (5)

I believe the answer is:

lying

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'not standing up' is the definition.
('lie' is the opposite to 'stand up')

I can't explain the rest of the clue.

(Other definitions for lying that I've seen before include "Perjury" , "Telling fibs" , "Not being disturbed" , "Invention" , "Prone (to fabrications?)" .)

I've seen this clue in The Guardian.

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