The most common and most likely answer to this crossword clue is the 8 letter word OBJECTOR
I've seen this crossword clue in The Times
O
B
J
E
C
T
O
R
More clues leading to the result OBJECTOR
- Dissident heading for jail in October, following revolution
- One challenging thing weighing on other ranks, initially
- See City peak after job changes one opposes
- One arguing against aim, bore at heart
- Somebody against men on purpose?
- . One protesting judge is involved in October revolution
- Target men? I don’t agree
- She opposes men on purpose
- One who won’t accept goal starts to offend referee
- Dissident died in Montmartre, first person caught on hill
Buddy explains!

The straight part
We know that "One challenging" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one challenging = objector.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 OBJECTOR mean?
OBJECTOR noun- A person who objects to something.