Could be butter or jam in full progress (2,1,4)

I believe the answer is:

on a roll

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If this is the answer I cannot explain how the clue works.

(Other definitions for on a roll that I've seen before include "Being extremely jammy, maybe" , "continually successful" , "Piano music was once stored here" , "Enjoying a winning streak" , "Where registered names are" .)

I've seen this clue in the Sydney Morning Herald.

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