Puzzle, conundrum (6)
I believe the answer is:
enigma
I believe this clue is a double definition.
'puzzle' is the first definition.
(I've seen this before)
'conundrum' is the second definition.
(synonyms)
(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" .)