Incontrovertible evidence (5)
I believe the answer is:
proof
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">incontrovertible evidence</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for proof that I've seen before include "Conclusive demonstration" , "Conclusive evidence or verification" , "Positive evidence" , "Render impervious (to)" , "whisky's strength?" .)