The most common and most likely answer to this crossword clue is the 9 letter word DIABOLISM
I've seen this crossword clue in Independent
D
I
A
B
O
L
I
S
M
Buddy explains!
The straight part
We know that "fiendish practice" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fiendish practice = diabolism.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 DIABOLISM mean?
DIABOLISM noun- Worship of the devil; Satanism
- Character, action, utterances, creative works, behavior or principles appropriate to the devil.
- Possession by Satan or other demonic forces.
- The property of having attributes that are demonic or Satanic.