Topple from power (4)
'topple from power' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oust that I've seen before include "Remove or reject from position" , "Throw out" , "Drive out, eject from office" , "Turn out" , "Put out, say of office" .)
(Other definitions for unseat that I've seen before include "Remove from power" , "Dislodge from saddle" , "Throw (as a 15?)" , "Get out of office" , "Remove from horse (or house?)" .)