Don't stand for an untruth (3)
I believe the answer is:
lie
'for an untruth' is the definition.
(lie is a kind of untruth)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for lie that I've seen before include "Fib in alienation" , "Perversion of the truth" , "False statement" , "situation" , "Perjure" .)