Puzzle, riddle (6)
I believe the answer is:
enigma
This clue is a double definition.
'puzzle' is the first definition.
(I've seen this before)
'riddle' is the second definition.
(I know that riddle can be written as enigma)
(Other definitions for enigma that I've seen before include "Solution to mystery" , "Mystery U puzzle" , "Mysterious puzzle" , "Gamine being a bit of a puzzle" , "WW2 code" .)