Seize without authority (5)
I believe the answer is:
usurp
'seize without authority' 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 "Oust" , "Commandeer" , "Assume by force" , "Supplant (someone in power)" , "Arrogate" .)