The most common and most likely answer to this crossword clue is the 14 letter word PRESUPPOSITION
I've seen this crossword clue in The Times
P
R
E
S
U
P
P
O
S
I
T
I
O
N
Buddy explains!
The straight part
We know that "Belief" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case belief = presupposition.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 PRESUPPOSITION mean?
PRESUPPOSITION noun- An assumption made beforehand; a preliminary conjecture or speculation.
- The act of presupposing.
- An assumption or belief implicit in an utterance or other use of language.