The most common and most likely answer to this crossword clue is the 8 letter word NAYSAYER
I've seen this crossword clue in Independent
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A
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More clues leading to the result NAYSAYER
- One obstinate and indeed singular philosopher
- One who refuses to snicker out loud at speaker
- One unwilling to support Northern queen after a year perhaps
- Any confusion? Horribly easy, right, for pessimist?
- Pessimist wasted many years losing money
- I oppose, in fact, state hospital drama
- One refuses any different speaker
Buddy explains!

The straight part
We know that "I oppose" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i oppose = naysayer.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 NAYSAYER mean?
NAYSAYER noun- One who consistently denies, criticizes, or doubts; a detractor.