Seize illegally (5)
I believe the answer is:
usurp
'seize illegally' 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 "Take control of without authority" , "Take over - misappropriate" , "Take (power) by force or illegally" , "Seize, take over" , "Wrongfully assume" .)