Conspirators (5)
'conspirators' is the definition.
('plotter' can be a synonym of 'conspirator')
This is all the clue.
(Other definitions for plotters that I've seen before include "People planning" , "eg, Guy Fawkes and co" .)
(Other definitions for cabal that I've seen before include "Caucus" , "Group of political intriguers" , "Group in plot" , "Group of conspirators" , "Secret group of plotters" .)