The most common and most likely answer to this crossword clue is the 10 letter word LIBERALISM
I've seen this crossword clue in Financial Times
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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 = liberalism.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 LIBERALISM mean?
LIBERALISM noun- The quality of being liberal.
- Any political movement founded on the autonomy and personal freedom of the individual, progress and reform, and government by law with the consent of the governed.
- An economic ideology in favour of laissez faire and the free market (related to economic liberalism).