Flat out, prepare for a fight (6,3)
I believe the answer is:
square off
'prepare for a fight' is the definition.
(square off can mean to assume a fighting pose)
'flat out' is the wordplay.
'flat' becomes 'square' (both can mean boring).
'out' becomes 'off' (I've seen this before).
'square'+'off'='SQUARE OFF'
(Other definitions for square off that I've seen before include "What boxer might do if attacked" , "Raise dukes" , "prepare for fight" .)