Go off in brotherhood (3)
I believe the answer is:
rot
'go off' is the definition.
(as in food going off)
'in brotherhood' is the wordplay.
'in' indicates a hidden word.
'ROT' is hidden within 'brotherhood'.
(Other definitions for rot that I've seen before include "'Decay, decompose (3)'" , "Rubbish (colloq.)" , "Twaddle" , "Informal nonsense" , "Gradual decline" .)