Long story a source of joy when upset (4)
I believe the answer is:
saga
'long story' is the definition.
(a saga is a long and complicated story)
'a source of joy when upset' is the wordplay.
'source of joy' becomes 'gas' (gas can mean something amusing or entertaining).
'when upset' shows that the letters should be reversed in order (in a down clue - set, written, up).
'a'+'gas'='agas'
'agas' in reverse letter order is 'SAGA'.
(Other definitions for saga that I've seen before include "Long heroic tale" , "Old story" , "Long involved tale" , "Norse narrative" , "Flag associated with a" .)