Gas giving energy to former nobleman (6)
I believe the answer is:
ethane
'gas' is the definition.
(I've seen this before)
'giving energy to former nobleman' is the wordplay.
'giving' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'to former nobleman' becomes 'thane'.
'e' put next to 'thane' is 'ETHANE'.
(Other definitions for ethane that I've seen before include "Ate hen (anag) - hydrocarbon gas" , "Flammable gas; eat hen (anagram)" , "Chemical" , "Athene (anag.); a colourless gas" , "Flammable hydrocarbon" .)