The most common and most likely answer to this crossword clue is the 9 plus 10 letter word BLOODLESS REVOLUTION
I've seen this crossword clue in The Times
B
L
O
O
D
L
E
S
S
R
E
V
O
L
U
T
I
O
N
Buddy explains!
The straight part
We know that "that led to James II’s downfall" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that led to james ii’s downfall = bloodless revolution.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.