Long story about a gas (4)
I believe the answer is:
saga
'long story' is the definition.
(a saga is a long and complicated story)
'about a gas' is the wordplay.
'about' says the letters should be written in reverse.
'a'+'gas'='agas'
'agas' backwards is 'SAGA'.
(Other definitions for saga that I've seen before include "Long detailed story" , "Lengthy tale" , "Long involved tale" , "Long and involved story" , "Long heroic tale" .)