Riddle that had many variations (6)
I believe the answer is:
'riddle' is the definition.
(I know that riddle can be written as enigma)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for enigma that I've seen before include "It's mysterious, difficult to understand" , "Title of Elgar's set of variations" , "'Puzzle, riddle (6)'" , "WW two Ger. code device; puzzle" , "German military cipher machine" .)