The most common and most likely answer to this crossword clue is the 7 letter word INFIDEL
I've seen this crossword clue in The Guardian
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More clues leading to the result INFIDEL
- Popular, Castro, but no Christian
- Find lie’s upset non-believer
- Non-believer in force led one to change direction
- Unbeliever elected Castro, say?
- Unbeliever me, suspicious fiend ending in hell
- He has no faith in candidates promoting Democrat
- Fielding’s wanton, no-good heathen
- He doesn’t believe in organised life to accommodate daughter
- Pagan with Manx feline, I’d suspect
- Where Cubans were asked to place their trust, I don’t believe
Buddy explains!

The straight part
We know that "Unbeliever" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unbeliever = infidel.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 INFIDEL mean?
INFIDEL noun- (now usually derogatory) One who does not believe in a certain religion.
- (now usually derogatory) One who does not believe in a certain principle.
- (now usually derogatory) One with no religious beliefs.
- Rejecting a specific religion.
- Of, characteristic of, or relating to unbelievers or unbelief.