Lie about in stench producing gas (6)
I believe the answer is:
helium
'gas' is the definition.
(I have seen 'Inert gas' mean 'helium' so perhaps 'gas' could also mean 'helium')
'lie about in stench' is the wordplay.
'about' is an anagram indicator.
'in' means one lot of letters goes inside another.
'stench' becomes 'hum' (I've seen this before).
'lie' is an anagram of 'eli'.
'eli' going within 'hum' is 'HELIUM'.
'producing' acts as a link.
(Other definitions for helium that I've seen before include "Second lightest element" , "Balloon-filling element" , "Light colourless gas, sometimes in balloons" , "Atomic number 2" , "Light colourless gas used in airships" .)