The most common and most likely answer to this crossword clue is the 11 letter word HIGH-FLYING
I've seen this crossword clue in Financial Times
H
I
G
H
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F
L
Y
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N
G
Buddy explains!
The straight part
We know that "successful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case successful = high-flying.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 HIGH-FLYING mean?
HIGH-FLYING adjective- Characteristic of a highflier; extravagant in conduct or opinion.