Tale about a gas (4)
I believe the answer is:
saga
'tale' is the definition.
(I've seen this before)
'about a gas' is the wordplay.
'about' says the letters should be written backwards.
'a'+'gas'='agas'
'agas' reversed gives 'SAGA'.
(Other definitions for saga that I've seen before include "A long adventurous story - it's a gas" , "Lengthy tale" , "Soap opera" , "A long story or poem - it's a gas" , "Account of great heroism" .)