Where Superman would be in his element (7)

I believe the answer is:

krypton

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'element' is the definition.
(krypton is a kind of element)

I don't understand how the rest of the clue works.

(Other definitions for krypton that I've seen before include "Superman's world and a gas" , "36 periodically?" , "Inert gas (symbol Kr)" , "Pry knot (anag) - gas" , "Chemical element, Kr" .)

I've seen this clue in The Independent.

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