The most common and most likely answer to this crossword clue is the 9 letter word INSURGENT
I've seen this crossword clue in The Guardian
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More clues leading to the result INSURGENT
- Mostly provide financial cover for decent chap and rebel
- Almost guarantee fellow is rebellious type
- Rebel in south demanding immediate action
- Rebel is compelling, knight admitted
- International partners joining desperate rebel
- Revolutionary new doctor dropping Oscar in it
- Rebel ruins revolutionary chap
- Rebel ruins corrupt fellow
- One sten gun shot bags resistance rebel
- Partisan from Gunners it turned out
Buddy explains!

The straight part
We know that "Rebel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rebel = insurgent.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 INSURGENT mean?
INSURGENT noun- One of several people who take up arms against the local state authority; a participant in insurgency.
- Rebellious, opposing authority.