The most common and most likely answer to this crossword clue is the 10 letter word EXCITATION
I've seen this crossword clue in The Daily Telegraph
E
X
C
I
T
A
T
I
O
N
Buddy explains!

The straight part
We know that "Fervour" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fervour = excitation.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 EXCITATION mean?
EXCITATION noun- The act of exciting or putting in motion; the act of rousing up or awakening.
- The act of producing excitement (stimulation); also, the excitement produced.
- The activity produced in an organ, tissue, or part, such as a nerve cell, as a result of stimulation
- The change in state as an excited state is formed by the absorption of a quantum of energy