Go off nonsense (3)
I believe the answer is:
rot
I believe this clue is a double definition.
'go off' is the first definition.
(as in food going off)
'nonsense' is the second definition.
(rot can mean nonsense or rubbish)
(Other definitions for rot that I've seen before include "Decay - rubbish" , "Collapse" , "Deteriorate" , "Balderdash" , "Cobblers" .)