Setter is both wordly and indestructible (8)
I believe the answer is:
immortal
'indestructible' is the definition.
(thesaurus)
'setter is both' is the wordplay.
'setter is' becomes 'I'm' (the setter is the person who set the crossword).
'both' becomes 'mortal' (I can't explain this - if you can you should give a lot more credence to this answer).
'im'+'mortal'='IMMORTAL'
'wordly and' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for immortal that I've seen before include "Imperishable" , "Deathless" , "Never dying" , "Everlasting" , "Great person" .)