Behave imperiously (8)
I believe the answer is:
domineer
'behave imperiously' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for domineer that I've seen before include "Ore mined badly could tend to overbear" , "Behave imperiously" , "Ride roughshod over" , "Browbeat" , "Boss around" .)