Crossword Genius

With "Don't you believe it!" go off (3)

Ross

I believe the answer is:

rot

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'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" .)

I've seen this clue in the Evening Standard.
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