Gas produces terrible groan (5)
I believe the answer is:
argon
'gas' is the definition.
(argon is a gas)
'terrible groan' is the wordplay.
'terrible' indicates anagramming the letters.
'groan' is an anagram of 'ARGON'.
'produces' acts as a link.
(Other definitions for argon that I've seen before include "something inert and insubstantial" , "Gas used in electric lights" , "elementary" , "Noble gas, atomic number 18" , "17 [GAS]" .)