Significant, momentous (8)
I believe the answer is:
historic
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">significant</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(of historic importance)</span><br/ ><br/ >'<span class="explanation-format__original">momentous</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for historic that I've seen before include "Epoch-making" , "sort of present" , "Pertaining to what is important in the past" , "Connecting with old times" , "Landmark; famous" .)