Defeat the greatest! (4)
I believe the answer is:
best
I believe this clue is a double definition.
'defeat' is the first definition.
(I know that best is a more specific form of beat)
'the greatest' is the second definition.
(I've seen this in another clue)
(Other definitions for best that I've seen before include "Of the highest quality" , "former football star" , "Overcome" , "The finest - worst" , "Superlative" .)