Significant event (9)
I believe the answer is:
milestone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">significant event</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for milestone that I've seen before include "Significant stage of development" , "Roadside distance marker" , "Important stage (reached)" , "Significant event or stage - a measure" , "Pillar" .)