Gas: average energy including couple of therms? (7)
I believe the answer is:
methane
'gas' is the definition.
(I've seen this before)
'average energy including couple of therms?' is the wordplay.
'average' becomes 'mean' (type of average).
'energy' becomes 'e' (physics symbol as in E=mch).
'including' indicates putting letters inside.
'couple of therms?' becomes 'th' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mean'+'e'='meane'
'meane' enclosing 'th' is 'METHANE'.
Can you help me to learn more?
(Other definitions for methane that I've seen before include "Main constituent of natural gas" , "Marsh-gas" , "Get fuel" , "Firedamp" , "compound" .)