Argument-settling fact (8)
I believe the answer is:
clincher
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">argument-settling fact</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 clincher that I've seen before include "Decisive factor" , "Decisive argument" , "Something decisive" , "Something settling a matter conclusively" , "Argument-settling fact" .)