The most common and most likely answer to this crossword clue is the 6 letter word GRISLY
I've seen this crossword clue in The Daily Telegraph
G
R
I
S
L
Y
More clues leading to the result GRISLY
Buddy explains!

The straight part
We know that "Frightful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case frightful = grisly.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 GRISLY mean?
GRISLY adjective- Horrifyingly repellent; gruesome, terrifying.
- In a horrible or terrible manner; in a terrifying way.