The most common and most likely answer to this crossword clue is the 9 letter word ATEMPORAL
I've seen this crossword clue in Financial Times
A
T
E
M
P
O
R
A
L
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The straight part
We know that "immortal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case immortal = atemporal.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ATEMPORAL mean?
ATEMPORAL adjective- Unaffected by time; timeless; permanent or unchanging.