Extraordinary puzzle! Arsenic extracted from powdered magnesia? (6)

I believe the answer is:

enigma

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'extraordinary puzzle arsenic' is the definition.
I know that 'puzzle' relates to this answer. I also know that both the answer and definition are singular nouns

I cannot understand how the rest of the clue works.

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(Other definitions for enigma that I've seen before include "Conundrum" , "Puzzle of gamine (6)," , "Mysteriously puzzling person or thing" , "Puzzling person/thing" , "Mysterious or puzzling thing" .)

I've seen this clue in The Hindu.

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