Only an extremely rare gas (6)
I believe the answer is:
butane
'rare gas' is the definition.
'butane' can be an answer for 'gas' (I have seen 'Flammable gas ' mean 'butane' so perhaps 'gas' could also mean 'butane'). I am not sure about the 'rare' bit.
'only an extremely' is the wordplay.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'extremely' becomes 'e' (this might be a standard abbreviation which I don't know about).
'but'+'an'+'e'='BUTANE'
Can you help me to learn more?
(Other definitions for butane that I've seen before include "Lighter fuel" , "Inflammable gas produced from petrol" , "Bottled gas used as a fuel" , "Flammable gas in petroleum" .)