Gas — more than enough but not all of it (6)
I believe the answer is:
ethane
'gas' is the definition.
(ethane is a gas)
'more than enough but not all of it' is the wordplay.
'not all of it' indicates a hidden word (only some of the letters are needed).
'ETHANE' is hidden within 'more than enough but'.
(Other definitions for ethane that I've seen before include "Fuel" , "Athene (anag.); a colourless gas" , "Compound" , "Ate hen (anag) - hydrocarbon gas" , "Flammable gas; eat hen (anagram)" .)