Series of electro-mechanical rotor cipher machines invented by German engineer Arthur Scherbius at the end of World War I (6)
I believe the answer is:
enigma
(Other definitions for enigma that I've seen before include "German military cipher machine" , "Puzzle found in gem, Ian" , "Puzzle of gamine (6)," , "Puzzling person/thing" , "It's mysterious, difficult to understand" .)