Take power unlawfully (5)
I believe the answer is:
usurp
'take power unlawfully' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for usurp that I've seen before include "Seize power without authority" , "Assume by force" , "Take possession of, illegally" , "Seize (throne, e.g.)" , "Arrogate" .)