Gas ring, note, for repair (8)
I believe the answer is:
nitrogen
'gas' is the definition.
(thesaurus)
'ring note for repair' is the wordplay.
'for repair' indicates anagramming the letters (I've seen this in other clues).
'ring'+'note'='ringnote'
'ringnote' anagrammed gives 'NITROGEN'.
(Other definitions for nitrogen that I've seen before include "It forms about four-fifths of atmosphere" , "It's a gas, the strange ringtone" , "Gas forming about four fifths of the atmosphere" , "chemists have fixation with?" , "creates much atmosphere" .)