Secret plot (6)
(Other definitions for scheme that I've seen before include "Design" , "Project or strategy" , "Lay plans" , "Machination" , "Plan - intrigue" .)
'secret plot' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cabal that I've seen before include "Small party" , "Secret plot or the people engaged in it" , "Clique of plotters" , "Plotting group" , "Secret plot; clique" .)