It's not to be believed (3)
I believe the answer is:
lie
'it's not to be believed' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lie that I've seen before include "Fib - be prone" , "Prevaricate or recline" , "Stay hidden" , "Porky" , "Rest; falsehood" .)