Long story is something exciting when written up (4)
I believe the answer is:
saga
'long story' is the definition.
(a saga is a long and complicated story)
'something exciting when written up' is the wordplay.
'something exciting' becomes 'ag' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'written up' is a reversal indicator (in a down clues, letters go up).
'ag'+'as'='agas'
'agas' reversed gives 'SAGA'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for saga that I've seen before include "Long heroic, family, story" , "Epic story" , "Long narrative" , "Long and involved story" , "Account of great heroism" .)