The most common and most likely answer to this crossword clue is the 11 letter word OPPORTUNITY
I've seen this crossword clue in Financial Times
O
P
P
O
R
T
U
N
I
T
Y
Buddy explains!

The straight part
We know that "Big break" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case big break = opportunity.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 OPPORTUNITY mean?
OPPORTUNITY noun- A chance for advancement, progress or profit.
- A favorable circumstance or occasion.
- (Euro-English) opportuneness