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.)" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">confirms</span>' is the definition.<br/ ><span class="explanation-format__explanation">(proving is a kind of confirming)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for proves that I've seen before include "Verifies" , "Provides evidence for" .)