The most common and most likely answer to this crossword clue is the 8 letter word DREADFUL
I've seen this crossword clue in The Guardian
D
R
E
A
D
F
U
L
More clues leading to the result DREADFUL
- Horrible daughter almost satisfied after study
- First impression of Dick Fuld era? Shockingly terrible!
- Frightful child read full-length extract
- Doctor led fraud? That’s appalling!
- Very bad medic in East End — he had flu “cured”
- Led astray with fraud? That’s terrible
- Drive feudal changes? That’s very bad
- Fraud led criminal to become very bad
- Terrible lad Freud sorted out
- Queen in a fuddle? Bad — very bad
Buddy explains!

The straight part
We know that "appalling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case appalling = dreadful.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 DREADFUL mean?
DREADFUL noun- A shocker: a report of a crime written in a provokingly lurid style.
- A journal or broadsheet printing such reports.
- A shocking or sensational crime.
- Full of something causing dread, whether
- Full of dread, whether
- Dreadfully.