Unique event (3-3)
I believe the answer is:
one-off
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unique event</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 one-off that I've seen before include "Unique thing" , "Not repeated, being a single event or object" , "Item made once only" , "Happening only once" , "Something unique" .)