Puzzle, conundrum (6)
I believe the answer is:
enigma
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">puzzle</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">conundrum</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for enigma that I've seen before include "There's a mystery in game" , "'Mystery, riddle (6)'" , "WW2 code" , "It's mysterious, difficult to understand" , "Something mysterious and puzzling" .)