The most common and most likely answer to this crossword clue is the 9 letter word MONSTROUS
I've seen this crossword clue in Financial Times
M
O
N
S
T
R
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S
More clues leading to the result MONSTROUS
Buddy explains!
The straight part
We know that "Horrible" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case horrible = monstrous.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 MONSTROUS mean?
MONSTROUS adjective- Hideous or frightful.
- Enormously large.
- Freakish or grotesque.
- Of, or relating to a mythical monster; full of monsters.
- Marvellous; strange.