Has eldest turned out to be indestructible? (9)
I believe the answer is:
deathless
'to be indestructible?' is the definition.
(thesaurus)
'has eldest turned' is the wordplay.
'turned' indicates an anagram.
'has'+'eldest'='haseldest'
'haseldest' with letters rearranged gives 'DEATHLESS'.
'out' acts as a link.
(Other definitions for deathless that I've seen before include "living forever" , "Never-ending" , "immortal" , "Lest heads, oddly, become everlasting" .)