Demonstrate conclusively (5)
I believe the answer is:
prove
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">demonstrate conclusively</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for prove that I've seen before include "Demonstrate beyond doubt" , "Ascertain" , "Demonstrate to be true" , "Demonstrate conclusively" , "Verify" .)