Strange thing (6)
I believe the answer is:
oddity
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">strange thing</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 oddity that I've seen before include "Strange person/thing" , "Peculiarity" , "Curious item" , "Something peculiar, person or event" , "one's mathematical quality?" .)