The most common and most likely answer to this crossword clue is the 8 letter word BUDDHIST
I've seen this crossword clue in The Guardian
B
U
D
D
H
I
S
T
More clues leading to the result BUDDHIST
Buddy explains!

The straight part
We know that "Believer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case believer = buddhist.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.