Final, conclusive (8)
I believe the answer is:
ultimate
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">final</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">conclusive</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for ultimate that I've seen before include "The last element of a series" , "Last or final in a series or process" , "Conclusive" , "Happening at the end of a process, final" , "Last or highest" .)