Of great significance (9)
I believe the answer is:
momentous
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of great significance</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 momentous that I've seen before include "of great significance" , "major" .)