The most common and most likely answer to this crossword clue is the 6 letter word THEIST
I've seen this crossword clue in The Guardian
T
H
E
I
S
T
More clues leading to the result THEIST
- Article first put together for believer
- Tourist's first hold-up, one believes
- Person of faith, one to probe my dalliance on vacation?
- Article by upright character, good person who believes in God
- The first sort of believer
- One believes he is only seen once in the robbery
- One who believes in starting to think before robbery
- Troop leader to burgle believer
- The ultimate in daylight robbery for a believer
- The first non-heathen
Buddy explains!

The straight part
We know that "One believes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one 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.