It's a mystery (6)
I believe the answer is:
enigma
'it's a mystery' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for enigma that I've seen before include "It's mysterious, difficult to understand" , "'Mystery, riddle (6)'" , "Puzzle of gamine (6)," , "Solution to mystery" , "It's mysterious like gamine" .)