The most common and most likely answer to this crossword clue is the 7 letter word DEMONIC
I've seen this crossword clue in The Times
D
E
M
O
N
I
C
More clues leading to the result DEMONIC
- Source of dubious income turned out to be fiendish
- Diabolical daughter has come in drunk
- Devilish fifth grade male disrupts teacher in charge
- Fiendish - half carrying on with crossword initially
- Hellish day I took part in last month
- Wicked Klan leader escaping from jail after protest
- Far from holy day I found near Christmas?
- Evil daughter has come in sozzled
- Duke with income set free of malign influence
- Furious, perhaps, having serving of cappuccinoupended
Buddy explains!

The straight part
We know that "Fiendish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fiendish = demonic.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 DEMONIC mean?
DEMONIC adjective- Pertaining to demons or evil spirits; demoniac.
- Pertaining to dæmons in ancient Greek thought; concerning supernatural ‘genius’.