The most common and most likely answer to this crossword clue is the 8 letter word HAIRIEST
I've seen this crossword clue in The Daily Telegraph
H
A
I
R
I
E
S
T
Buddy explains!
The straight part
We know that "Most frightening" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most frightening = hairiest.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 HAIRIEST mean?
HAIRIEST adjective- Of a person, having a lot of hair on the body.
- Of an animal, having a lot of fur.
- Of a body part other than the head, having hair growing from it.
- Difficult, complex, intricate, or intimidating.
- Causing anxiety or fright; terrifying, scary.