The most common and most likely answer to this crossword clue is the 7 plus 1 letter word THEIST} :
I've seen this crossword clue in The Daily Telegraph
T
H
E
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:
Buddy explains!
The straight part
We know that "One who believes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one who believes = theist} :.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 THEIST} : mean?
THEIST} : noun- One who believes in the existence of a god or gods.