The most common and most likely answer to this crossword clue is the 8 letter word FEMINIST
I've seen this crossword clue in Financial Times
F
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More clues leading to the result FEMINIST
Buddy explains!

The straight part
We know that "campaigner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case campaigner = feminist.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 FEMINIST mean?
FEMINIST noun- An advocate of feminism; a person who believes in bringing about the equality of the sexes (of women and men) in all aspects of public and private life
- A member of a feminist political movement
- Relating to or in accordance with feminism.