Widespread occurrence as already mentioned in a long poem (8)
I believe the answer is:
epidemic
'widespread' is the definition.
(I've seen this in another clue)
'mentioned in a long poem' is the wordplay.
'mentioned' becomes 'idem' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'a long poem' becomes 'epic' (I've seen this in another clue).
'idem' inserted inside 'epic' is 'EPIDEMIC'.
'occurrence as already' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for epidemic that I've seen before include "medical emergency" , "lot of disease" , "Plague" , "health emergency" , "Citrus fruit" .)