Conclusive evidence (5)
I believe the answer is:
proof
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">conclusive evidence</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 proof that I've seen before include "Conclusive demonstration" , "Testimony" , "Trial printing" , "mathematical work" , "whisky's strength?" .)