Superman's world is gas (7)

I believe the answer is:

krypton

(Other definitions for krypton that I've seen before include "Noble gas, Kr" , "Superman's world and a gas" , "Inert gas (symbol Kr)" , "Inert gas used in florescent lights (7)" , "Pry knot (anag) - gas" .)

I've seen this clue in The Irish Times.

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