The most common and most likely answer to this crossword clue is the 11 letter word EVANGELICAL
I've seen this crossword clue in The Times
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More clues leading to the result EVANGELICAL
Buddy explains!

The straight part
We know that "Fervent believer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fervent believer = evangelical.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 EVANGELICAL mean?
EVANGELICAL noun- A member of an evangelical church
- An advocate of evangelicalism
- Pertaining to the doctrines or teachings of the Christian gospel or Christianity in general.
- Pertaining to the gospel(s) of the Christian New Testament.
- Protestant; specifically Lutheran and Calvinist churches in continental Europe as well as their offshoots in North America.
- Pertaining to a movement in Protestant Christianity that stresses personal conversion and the authority of the Bible (evangelicalism).
- Pertaining to Islamic groups that are dedicated to dawah and preaching the Quran and sunnah.
- Zealously enthusiastic.