Go off regularly on time, always at the start (3)

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 rest of the clue works.

(Other definitions for rot that I've seen before include "Drivel" , "Decay or disintegrate" , "Deteriorate" , "Rhubarb" , "Rubbish (colloq.)" .)

I've seen this clue in The Times.

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