Short ballad a mysterious writer pens is revelation (10)
I believe the answer is:
apocalypse
'is revelation' is the definition.
(I've seen this before)
'short ballad a mysterious writer pens' is the wordplay.
'short' means to remove the last letter.
'ballad' becomes 'calypso' (I've seen this before).
'mysterious writer' becomes 'poe' (Edgar Allan Poe. I am not sure about the 'mysterious' bit.).
'pens' means one lot of letters goes inside another (to pen can mean to contain or imprison).
'calypso' with its last letter taken away is 'calyps'.
'a'+'poe'='apoe'
'calyps' inserted into 'apoe' is 'APOCALYPSE'.
Can you help me to learn more?
(Other definitions for apocalypse that I've seen before include "Violent event" , "World's end" , "terrible event" , "End of the world" , "Catastrophic event" .)