Where butter may be found in the groove (2,1,4)
I believe the answer is:
on a roll
'in the groove' is the definition.
I cannot understand how the remainder of the clue works.
(Other definitions for on a roll that I've seen before include "How butter may be spread" , "Where one may find sesame seeds" , "continually successful" , "Being extremely jammy, maybe" , "Going through a good patch" .)