Mystery, puzzle (6)
I believe the answer is:
enigma
I believe this clue is a double definition.
'mystery' is the first definition.
(I know that mystery can be written as enigma)
'puzzle' is the second definition.
(I've seen this in another clue)
(Other definitions for enigma that I've seen before include "Title of Elgar's set of variations" , "It's mysterious like gamine" , "Mysterious puzzle" , "'Puzzle, riddle (6)'" , "Solution to mystery" .)