Ignore heartless nut spouting gas (8)
I believe the answer is:
nitrogen
'gas' is the definition.
(I know that nitrogen is a type of gas)
'ignore heartless nut spouting' is the wordplay.
'heartless' means to remove the middle letters.
'spouting' indicates anagramming the letters.
'nut' with its centre removed is 'nt'.
'ignore'+'nt'='ignorent'
'ignorent' is an anagram of 'NITROGEN'.
(Other definitions for nitrogen that I've seen before include "It constitutes 78 percent of the atmosphere" , "Gas forming about four fifths of the atmosphere" , "It forms about four-fifths of atmosphere" , "Neon grit (anag.)" , "Much of what we take in" .)