The most common and most likely answer to this crossword clue is the 5 letter word DEISM
I've seen this crossword clue in The Guardian
D
E
I
S
M
Buddy explains!
The straight part
We know that "Fundamental belief" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fundamental belief = deism.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 DEISM mean?
DEISM noun- A philosophical belief in the existence of a god (or goddess) knowable through human reason; especially, a belief in a creator god unaccompanied by any belief in supernatural phenomena or specific religious doctrines.
- Belief in a god who ceased to intervene with existence after acting as the cause of the cosmos.