The most common and most likely answer to this crossword clue is the 9 letter word APOCRYPHA
A
P
O
C
R
Y
P
H
A
More clues leading to the result APOCRYPHA
- Some books: each had brief, overwhelming appeal
- Books having Pilot Officer call for learner in top grade
- Surprisingly happy with old car and rejected books
- Statements yet to be validated, a policeman backing our story up with data, finally
- Strange approach taken about year books
- Fresh approach covering the ultimate in cookery books
- Approach rear of abbey to move religious works
- Set of religious books that could make chap pray, having hidden love
- A cheer burst out having started books
- Head of Apple deliriously happy restriction of reading text works
Buddy explains!

The straight part
We know that "they’re not necessarily believed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they’re not necessarily believed = apocrypha.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 APOCRYPHA mean?
APOCRYPHA noun- Something, as a writing, that is of doubtful authorship or authority (formerly also used attributively).