Confirms (6)
(Other definitions for agrees that I've seen before include "Shakes hands" , "Accedes or concurs" , "Is of one mind" , "Holds the same opinion" , "Grease (anag.)" .)
'confirms' is the definition.
(proving is a kind of confirming)
This is all the clue.
(Other definitions for proves that I've seen before include "Verifies" , "Provides evidence for" .)