Flourish or wave as a threat (8)
I believe the answer is:
brandish
This clue is a double definition.
'flourish' is the first definition.
(I know that flourish can be written as brandish)
'wave as a threat' is the second definition.
(I've seen this before)
(Other definitions for brandish that I've seen before include "Flourish aggressively" , "Wave like a weapon" , "Exhibit aggressively" , "Display" , "shake" .)