Talk about a long story! (4)
I believe the answer is:
saga
'long story' is the definition.
(a saga is a long and complicated story)
'talk about a' is the wordplay.
'talk' becomes 'gas' (gas can mean to chat).
'about' says the letters should be written backwards.
'gas' written backwards gives 'sag'.
'sag'+'a'='SAGA'
(Other definitions for saga that I've seen before include "Long and involved story" , "A long story or poem - it's a gas" , "Lengthy tale" , "Long heroic, family, story" , "Epic story" .)