The most common and most likely answer to this crossword clue is the 8 letter word PACIFIST
I've seen this crossword clue in The Times
P
A
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More clues leading to the result PACIFIST
- One won’t fight treaty
- Anti-war literary genre getting no opening in time gone by
- Anti-war protester curtailed agreement, one with weapon in hand?
- Non-belligerent condition is enshrined in treaty
- Contract will cover as long as one’s not 21
- Condition is put into agreement for one seeking to avoid conflict
- Not advocating war, if one’s in treaty
- Not a warmonger, provided one’s bound by treaty
- Revolutionary individual originally associated with top duke, one avoiding struggle
- Non-combatant, provided one's in agreement
Buddy explains!

The straight part
We know that "Anti-war protester" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case anti-war protester = pacifist.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 PACIFIST mean?
PACIFIST noun- One who loves, supports, or favours peace.
- One who prefers to avoid violence.
- One who opposes violence and is anti-war.
- A player who attempts the challenge of winning a game without attacking any enemy characters.
- Of or relating to pacifism.