Scottish chieftain going after English — that's a gas! (6)
I believe the answer is:
ethane
'that's a gas' is the definition.
(ethane is a gas)
'scottish chieftain going after english' is the wordplay.
'scottish chieftain' becomes 'thane' (historical Scottish rank).
'going after' is a charade indicator (letters next to each other).
'english' becomes 'E' (abbreviation as in OED).
'thane' put after 'e' is 'ETHANE'.
(Other definitions for ethane that I've seen before include "Fuel" , "Compound" , "Chemical" , "Flammable hydrocarbon" , "Flammable gas; eat hen (anagram)" .)