Long story about a very enjoyable occasion (4)
I believe the answer is:
saga
'long story' is the definition.
(a saga is a long and complicated story)
'about a very enjoyable occasion' is the wordplay.
'about' says the letters should be written in reverse.
'very enjoyable occasion' becomes 'gas' (I can't explain this - if you can you should believe this answer much more).
'a'+'gas'='agas'
'agas' in reverse letter order is 'SAGA'.
Can you help me to learn more?
(Other definitions for saga that I've seen before include "Medieval Icelandic legend" , "Long narrative" , "A long story or poem - it's a gas" , "Old story" , "Lengthy tale" .)