Puzzling thing (6)
I believe the answer is:
enigma
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">puzzling 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 enigma that I've seen before include "WWII cipher" , "Mysteriously puzzling person or thing" , "Riddle - German coding machine" , "Title of Elgar's set of variations" , "Gamine being a bit of a puzzle" .)