The most common and most likely answer to this crossword clue is the 9 letter word FIFTEENTH
I've seen this crossword clue in Financial Times
F
I
F
T
E
E
N
T
H
Buddy explains!
The straight part
We know that "Two octaves" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case two octaves = fifteenth.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 FIFTEENTH mean?
FIFTEENTH noun- The person or thing in the fifteenth position.
- One of fifteen equal parts of a whole.
- The interval comprising two octaves.
- The ordinal form of the number fifteen.