Not all authors are immortal (4)
I believe the answer is:
thor
'immortal' is the definition.
(Thor is an example)
'not all authors are' is the wordplay.
'not all' says the answer is hidden in the clue.
'THOR' is hidden in the letters of 'authors are'.
(Other definitions for thor that I've seen before include "The Thunderer" , "Mythical figure" , "Thunderous god" , "Norse hammer-wielding god" , "can be classed as immortal" .)