Remove from political office (6)
(Other definitions for unseat that I've seen before include "Get out of office" , "Throw (as a 15?)" , "Dislodge from saddle" , "Cause to fall from the saddle" , "Remove from horse (or house?)" .)
(Other definitions for oust that I've seen before include "Turn out" , "Give marching orders" , "Get rid of" , "Remove or reject from position" , "Extract" .)