If it is, don't you believe it (6)
I believe the answer is:
untrue
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for untrue that I've seen before include "Not in accordance with fact" , "not right!" , "Incorrect or false" , "Not accurate or veracious" , "Counter-factual" .)