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 a hidden word.
'TALE' can be found hidden inside '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" .)