The most common and most likely answer to this crossword clue is the 9 letter word PROTESTER
I've seen this crossword clue in The Daily Telegraph
P
R
O
T
E
S
T
E
R
More clues leading to the result PROTESTER
- Harry eats rubbish? I object!
- One who demonstrates in favour of examiner
- Person paid by examination board? I object
- One in favour of street rioting?
- One objecting to 50% reduction in proper sample
- One who demonstrates corruption getting into trouble
- He objects to badger being fed rubbish
- Annoyance about rubbish over Queen objector
- See 3d
- One who demonstrates in favour of someone doing experiments?
Buddy explains!

The straight part
We know that "One who demonstrates" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one who demonstrates = protester.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 PROTESTER mean?
PROTESTER noun- One who protests, either singly or in a public display of group feeling.
- One who protests a bill of exchange, or note.