Jabber about a story (4)
I believe the answer is:
saga
'story' is the definition.
(saga is a kind of story)
'jabber about a' is the wordplay.
'jabber' becomes 'gas' (both can mean to talk excessively).
'about' says the letters should be written backwards.
'gas' in reverse letter order is 'sag'.
'sag'+'a'='SAGA'
(Other definitions for saga that I've seen before include "Epic story" , "Tale of heroes" , "Long and involved story" , "Heroic narrative" , "A long story or poem - it's a gas" .)