Short ballad penned by a writer in World's End (10)
I believe the answer is:
apocalypse
'world's end' is the definition.
(the end of the world)
'short ballad penned by a writer in' is the wordplay.
'short' means to remove the last letter.
'ballad' becomes 'calypso' (I've seen this before).
'penned by' indicates putting letters inside (inserted letters are penned in).
'writer in' becomes 'poe' (I can't explain this - if you can you should give a lot more credence to this answer).
'calypso' with its last letter removed is 'calyps'.
'a'+'poe'='apoe'
'calyps' inserted inside 'apoe' is 'APOCALYPSE'.
(Other definitions for apocalypse that I've seen before include "terrible event" , "End of the world" , "the last item" , "Catastrophic event" , "This last book" .)