The most common and most likely answer to this crossword clue is the 10 letter word DAMNATION!
I've seen this crossword clue in The Guardian
D
A
M
N
A
T
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!
Buddy explains!
The straight part
We know that "It’s hell" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s hell = damnation!.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 DAMNATION! mean?
DAMNATION! noun- The state of being damned; condemnation; openly expressed disapprobation.
- Condemnation to everlasting punishment in the future state, or the punishment itself.