The most common and most likely answer to this crossword clue is the 7 letter word MONSTER
I've seen this crossword clue in Financial Times
M
O
N
S
T
E
R
More clues leading to the result MONSTER
- Enormous characters in gym, on steroids
- Ogre featured in sermons — terrible!
- Foul fiend featured in sermon’s terrifying
- Rolling Stone caught by Mr Big? Very!
- See 19
- Keeping on bad terms is a horrible thing
- Tremendous battle, lost cause, never ends
- Very big day before relaxing rest
- Beast in beret’s no musical backing
- Scary figure in gym on steroids
Buddy explains!

The straight part
We know that "a horrible thing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a horrible thing = monster.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 MONSTER mean?
MONSTER noun- A terrifying and dangerous creature.
- A bizarre or whimsical creature.
- A cruel or antisocial person, especially a criminal.
- A horribly deformed person.
- A badly behaved child, a brat.
- Something unusually large.
- A prodigy; someone very talented in a specific domain.
- A non-player character that player(s) fight against in role-playing games.
- To make into a monster; to categorise as a monster; to demonise.
- To behave as a monster to; to terrorise.
- To harass.
- Very large; worthy of a monster.
- Great; very good; excellent.