Something hard to understand or explain (6)

I believe the answer is:

enigma

(Other definitions for enigma that I've seen before include "Mysterious or puzzling thing" , "Mysterious puzzle" , "It's mysterious like gamine" , "WW two Ger. code device; puzzle" , "Elgar Variations nickname" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.