Riddle that had many variations (6)

I believe the answer is:

enigma

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'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" .)

I've seen this clue in The Times.

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