Not to be disputed (16)
I believe the answer is:
incontrovertible
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not to be disputed</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 incontrovertible that I've seen before include "certain" , "Not to be denied" , "Not to be disputed" , "Proven" .)