Cold gas, natural (9)
I believe the answer is:
heartless
'cold' is the definition.
(thesaurus)
'gas natural' is the wordplay.
'gas' becomes 'He' (chemical symbol for helium).
'natural' becomes 'artless' (I've seen this before).
'he'+'artless'='HEARTLESS'
(Other definitions for heartless that I've seen before include "hard?" , "Scrooge was" , "Lacking all feelings" , "Lacking feeling through cardiac deficiency" , "That's very cruel" .)