The most common and most likely answer to this crossword clue is the 5 letter word JIHAD
I've seen this crossword clue in The Daily Telegraph
J
I
H
A
D
More clues leading to the result JIHAD
Buddy explains!

The straight part
We know that "fervent crusade" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fervent crusade = jihad.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 JIHAD mean?
JIHAD noun- A holy war undertaken by Muslims.
- An aggressive campaign for an idea.
- A personal spiritual struggle for self-improvement and/or against evil.
- To participate in a jihad.