Seize unlawfully (5)
I believe the answer is:
usurp
'seize unlawfully' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for usurp that I've seen before include "Wrongfully assume" , "Take over - misappropriate" , "Assume authority" , "Take power unlawfully" , "Take (power) by force or illegally" .)