Puzzling person or thing (6)
I believe the answer is:
enigma
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">puzzling person</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">thing</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I have seen 'Mysterious thing ' mean 'enigma' so perhaps 'thing' could also mean 'enigma')</span><br/ ><br/ ></span>
(Other definitions for enigma that I've seen before include "Mysterious person/thing" , "WW2 code" , "Title of Elgar's set of variations" , "Mysterious person" , "cryptic puzzle!" .)