Gas produced by a defective organ (5)
I believe the answer is:
argon
'gas' is the definition.
(thesaurus)
'a defective organ' is the wordplay.
'a defective' is an anagram indicator.
'organ' is an anagram of 'ARGON'.
'produced by' is the link.
(Other definitions for argon that I've seen before include "Organ (anag.)" , "Inert gas, symbol Ar" , "Noble gas, Ar" , "Inert eighteen ac gas" , "noble element" .)