Gas ring on with heat evenly spread (8)
I believe the answer is:
nitrogen
'gas' is the definition.
(I know that nitrogen is a type of gas)
'ring on with heat evenly spread' is the wordplay.
'with' says to put letters next to each other.
'evenly' means one should take alternating letters (the letters in even-numbered positions).
'spread' indicates anagramming the letters.
The alternating letters of 'heat' are 'et'.
'ring'+'on'='ringon'
'ringon' put after 'et' is 'etringon'.
'etringon' with letters rearranged gives 'NITROGEN'.
(Other definitions for nitrogen that I've seen before include "Chemical element" , "It forms about four-fifths of atmosphere" , "that's dominant in the air" , "It makes up 78% of air" , "Commonest atmospheric gas" .)