Systematic plan (6)
'systematic plan' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for scheme that I've seen before include "Secret plan, plot" , "proposal" , "Project or strategy" , "Lay plans" , "'Plot, plan (6)'" .)
(Other definitions for regimen that I've seen before include "Prescribed course of diet, exercise, etc" , "Method of working" , "Course of diet, therapy" , "Recommended course" , "Prescribed system of diet and exercise - mere gin (anag)" .)