The most common and most likely answer to this crossword clue is the 8 letter word AGNOSTIC
I've seen this crossword clue in The Guardian
A
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More clues leading to the result AGNOSTIC
- One doubts a costing is wrong
- A good number remain endlessly uncertain about what to believe
- Flipping fuss about jerk such as Doubting Thomas
- Acting so uncertain — is one?
- Sceptic coasting around
- One doesn’t believe it: number reversed in account
- Sceptical sort acting so atrociously
- Doubting it can go faster – just in third place, sadly
- A good number remain, we hear, doubtful
- Acting so strangely, not knowing what to believe
Buddy explains!

The straight part
We know that "O/ne doesn’t believe" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case o/ne doesn’t believe = agnostic.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 AGNOSTIC mean?
AGNOSTIC noun- A person who holds to a form of agnosticism, especially uncertainty of the existence of a deity.
- Of or relating to agnosticism or its adherents.
- Doubtful or uncertain about the existence or demonstrability of God or other deity.
- Of a software component etc.: unaware or noncommittal regarding the specific nature of the components or input with which it interacts.
- (usually with a prepositional phrase) Having no firmly held opinions on an issue or matter of uncertainty.