One seizing power (7)
I believe the answer is:
usurper
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one seizing power</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)