The most common and most likely answer to this crossword clue is the 8 letter word HUMANIST
I've seen this crossword clue in The Guardian
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More clues leading to the result HUMANIST
Buddy explains!
The straight part
We know that "one who rejects religion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one who rejects religion = humanist.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 HUMANIST mean?
HUMANIST noun- A scholar of one of the subjects in the humanities.
- A person who believes in the philosophy of humanism.
- In the Renaissance, a scholar of Greek and Roman classics.
- Relating to humanism or the humanities.
- Of a typeface: resembling classical handwritten monumental Roman letters rather than the 19th-century grotesque typefaces.