Immortal (9)
There are a few possibilities for this:
deathless
'immortal' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for eternal that I've seen before include "going on for ever" , "Not stopping" , "Lasting forever, tediously persistent" , "Everlasting, not affected by time" , "Forever and always" .)
'immortal' is the definition.
(similar in meaning)
This is the entire clue.
(Other definitions for undying that I've seen before include "Inextinguishable" , "Everlasting" , "Lasting forever, not subject to mortality" , "Constant" , "Immortal, eternal" .)
'immortal' is the definition.
(similar in meaning)
This is the entire clue.
(Other definitions for deathless that I've seen before include "immortal" , "living forever" , "Lest heads, oddly, become everlasting" , "Never-ending" .)