Behave overbearingly (8)
I believe the answer is:
domineer
'behave overbearingly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for domineer that I've seen before include "Browbeat" , "Ride roughshod over" , "behave arrogantly" , "Boss around" , "Exercise power over in an autocratic manner" .)