Behave imperiously (8)
I believe the answer is:
domineer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">behave imperiously</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)