With "Don't you believe it!" go off (3)
I believe the answer is:
rot
'go off' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rot that I've seen before include "Twaddle" , "Cobblers" , "Collapse" , "Corruption" , "Decay, wither" .)