The most common and most likely answer to this crossword clue is the 8 letter word NIHILIST
I've seen this crossword clue in The Times
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More clues leading to the result NIHILIST
- Popular revolutionary Henry is lit up? I deny everything!
- Nighy: Liszt reportedly “an absolute non-believer”
- Sort of philosopher in DUP territory greeting tilt to one side
- Supporter of 15 from Northern Ireland getting one heel past goal in rugby?
- Turning in targets for assassinwho’s very sceptical
- I instil Harry’s head with muddle and nothing I believe in
- Northern Ireland’s troubles start to leave a catalogue of victims, promoting profound negativity
- Something must be denied by this philosopher
- Greeting discovered in metal record is revolutionary
- Denier of values -- nickel, hydrogen, iodine, the whole roster
Buddy explains!

The straight part
We know that "I deny everything" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i deny everything = nihilist.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 NIHILIST mean?
NIHILIST noun- A person who accepts or champions nihilism.
- An absolute skeptic; a person who believes in the truth of nothing.