Number coming up with a story (4)
I believe the answer is:
saga
'story' is the definition.
(I know that saga is a type of heroic tale)
'number coming up with a' is the wordplay.
'number' becomes 'gas' (anaesthetic gas - something which numbs).
'coming up' says the letters should be written backwards.
'with' is a charade indicator (letters next to each other).
'gas' in reverse letter order is 'sag'.
'sag'+'a'='SAGA'
(Other definitions for saga that I've seen before include "Epic - legend" , "Tale of heroes" , "Long and involved story" , "Norse narrative" , "Long heroic tale" .)