The most common and most likely answer to this crossword clue is the 9 letter word PROFUSION
I've seen this crossword clue in Financial Times
P
R
O
F
U
S
I
O
N
Buddy explains!

The straight part
We know that "Extravagance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extravagance = profusion.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 PROFUSION mean?
PROFUSION noun- Abundance; the state of being profuse; a cornucopia
- Lavish or imprudent expenditure; prodigality or extravagance