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"campaigner" is also used as a crossword clue. If that's what you saw, the answer is often activist (8 letters).
Definitions
a politician who is running for public office
Word origin
from campaign (noun) + -er (occupational suffix).
Used in a sentence
“The US supreme court has ruled unanimously that natural human genes cannot be patented, a decision that scientists and civil rights campaigners said removed a major barrier to patient care and medical innovation.”
“It is remembered unfondly by politicians, communities and campaigners as the document that heralded widespread reductions in the size of the railway.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Cause advocate”10 letters
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