One may be troubled, somehow not trustful ultimately (7)

I believe the answer is:

doubter

I'm a little stuck... Click here to teach me more about this clue!

'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for doubter that I've seen before include "E.g. St Thomas" , "One who is not sure" , "one such as Thomas" .)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.