Changed ringtone element (8)
I believe the answer is:
nitrogen
'element' is the definition.
(nitrogen is a kind of element)
'changed ringtone' is the wordplay.
'changed' indicates anagramming the letters.
'ringtone' anagrammed gives 'NITROGEN'.
(Other definitions for nitrogen that I've seen before include "Main constituent of the air" , "Constituent of four fifths of earth's atmosphere" , "Commonest atmospheric gas" , "It forms the greater part of air" , "It makes up 78% of air" .)