Finished, might defeat (9)
I believe the answer is:
overpower
'defeat' is the definition.
(I know that overpower is a more specific form of the action beat)
'finished might' is the wordplay.
'finished' becomes 'over' (over can mean finished or ended).
'might' becomes 'power' (being mighty).
'over'+'power'='OVERPOWER'
(Other definitions for overpower that I've seen before include "Subdue - beat" , "Crush" , "Best" , "Conquer" , "Subjugate" .)