The most common and most likely answer to this crossword clue is the 7 letter word ATHEISM
I've seen this crossword clue in Financial Times
A
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More clues leading to the result ATHEISM
- Lack of belief in time has dissipated
- What He is mostly limiting?
- Bloke lives in hole in the wall, showing disbelief
- Lack of faith in so many leaders chasing articles
- Disbelief that two articles are supported by one manuscript that happened to turn up
- One in Thames struggling with unbelief
- Unbelief is conveyed in article by those people
- Lack of faith in humanity
- A lack of faith in his team, unfortunately
- Doctrine at odds with religious beliefs, shame it unravelled
Buddy explains!

The straight part
We know that "disbelief" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case disbelief = atheism.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 ATHEISM mean?
ATHEISM noun- (narrowly) Belief that no deities exist (sometimes including rejection of other religious beliefs).
- (broadly) Rejection of belief that any deities exist (with or without a belief that no deities exist).
- (very broadly) Absence of belief that any deities exist (including absence of the concept of deities).
- Absence of belief in a particular deity, pantheon, or religious doctrine (notwithstanding belief in other deities).