Story concealed by mortal enemies (4)
I believe the answer is:
tale
'story' is the definition.
(I know that story can be written as tale)
'concealed by mortal enemies' is the wordplay.
'concealed by' indicates the answer is hidden within the clue.
'TALE' is hidden in the letters of 'mortal enemies'.
(Other definitions for tale that I've seen before include "Shakespeare play: 'The Winter's . . . .'" , "Fable" , "A story, perhaps out of school" , "'Story, narrative (4)'" , "Story or anecdote" .)