The most common and most likely answer to this crossword clue is the 9 letter word MOMENTOUS
I've seen this crossword clue in Financial Times
M
O
M
E
N
T
O
U
S
More clues leading to the result MOMENTOUS
- Fateful sign all of the French associated with mass
- Having import from second country to carry round
- Significant warning sign to America, after end of Vietnam
- Significant American in pursuit of second Oscar
- Doctor, doctor! One must be serious!
- Major’s second American staying over
- Important opus oddly written within minutes
- Important sign to American after end of creationism
- Second American to chase ring of huge significance
- Ring us following shake of great significance
Buddy explains!

The straight part
We know that "Major" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case major = momentous.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 MOMENTOUS mean?
MOMENTOUS adjective- Outstanding in importance, of great consequence.