Illegally seize (5)
I believe the answer is:
usurp
'illegally seize' is the definition.
(eg usurping a throne)
This is the entire clue.
(Other definitions for usurp that I've seen before include "Appropriate without authority" , "Supplant (someone in power)" , "Take (power) by force or illegally" , "Seize power without authority" , "Take power unlawfully" .)