Could be butter or jam in full progress (2,1,4)
I believe the answer is:
on a roll
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(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" .)