Seizing control without authority (8)
I believe the answer is:
usurping
'seizing control without authority' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for usurping that I've seen before include "Taking another's position wrongfully" , "Taking power without authority" , "Seizing control without authority" , "Assuming power without authority" , "Seizing control illegally" .)