Gas ring not designed to save energy (8)
I believe the answer is:
nitrogen
'gas' is the definition.
(thesaurus)
'ring not designed to save energy' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'to save' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'ring'+'not'='ringnot'
'ringnot' is an anagram of 'nitrogn'.
'nitrogn' going around 'e' is 'NITROGEN'.
(Other definitions for nitrogen that I've seen before include "Inert nog is an inert gas" , "Main gas in air" , "Constituent of four fifths of earth's atmosphere" , "It constitutes 78 percent of the atmosphere" , "Main element of air" .)