Remove, as a ruler (6)
(Other definitions for oust that I've seen before include "Expel from office or position" , "Give marching orders" , "kick out" , "Drive out, eject from office" , "Send packing" .)
(Other definitions for depose that I've seen before include "Remove from power or testify in court" , "Remove from a high position" , "Remove from office or position" , "Oust or unseat" , "Remove from office; testify" .)