One seizing power (7)
I believe the answer is:
usurper
'one seizing power' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for usurper that I've seen before include "One seizes power illegally" , "One taking position by force" , "Pursuer of person who grabs power" , "One who takes power or position wrongfully" , "One wrongfully seizes power from another" .)