Unique thing (3-3)
I believe the answer is:
one-off
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unique thing</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 one-off that I've seen before include "Unique thing" , "Not repeated - singular" , "Unique specimen" , "Not repeated or reproduced, a single thing" , "A single event, not reproduced or repeated" .)