Epic story about a gas (4)
I believe the answer is:
saga
'epic story' is the definition.
(I've seen this in another clue)
'about a gas' is the wordplay.
'about' says the letters should be written backwards.
'a'+'gas'='agas'
'agas' written backwards gives 'SAGA'.
(Other definitions for saga that I've seen before include "Body of legend" , "Long heroic tale" , "Long and involved story" , "Long involved tale" , "Epic story" .)