Speed to defeat (4)
I believe the answer is:
lick
I believe this is a double definition.
'speed' is the first definition.
(I've seen this before)
'defeat' is the second definition.
(lick can mean to beat or defeat)
'to' is the link.
(Other definitions for lick that I've seen before include "Defeat (informal) - speed (informal)" , "Pass tongue over" , "Beat; vigorous speed" , "compressed salt for domestic animals" , "Moisten" .)