The most common and most likely answer to this crossword clue is the 9 letter word REBELLION
I've seen this crossword clue in The Guardian
R
E
B
E
L
L
I
O
N
More clues leading to the result REBELLION
- Bishop in staggering movement, hero in act of defiance
- Dissent about call upset chief
- Rising wind traps black beast
- Beer off? Flipping poorly on rising
- A wildcat rising?
- British rugby player behind guerrilla insurgence
- Boxer, maybe, with electronic phone featured in retrograde film genre
- Rising, like a defiant beast
- Bishop in wobble with hero in defiance
- Fizzy beer left by beast — revolting
Buddy explains!
The straight part
We know that "act of defiance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case act of defiance = rebellion.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 REBELLION mean?
REBELLION noun- Armed resistance to an established government or ruler.
- Defiance of authority or control; the act of rebelling.
- An organized, forceful subversion of the law of the land in an attempt to replace it with another form of government.