The most common and most likely answer to this crossword clue is the 7 letter word SKEPTIC
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More clues leading to the result SKEPTIC
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 = skeptic.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 SKEPTIC mean?
SKEPTIC noun- Someone who habitually doubts beliefs and claims presented as accepted by others, requiring strong evidence before accepting any belief or claim.
- Someone undecided as to what is true.
- A type of agnostic; someone skeptical towards religion.