Where Superman would be in his element (7)
I believe the answer is:
krypton
'element' is the definition.
(krypton is a kind of element)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(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" .)