The most common and most likely answer to this crossword clue is the 6 plus 2 letter word SEPTET SE
S
E
P
T
E
T
S
E
Buddy explains!
The straight part
We know that "musical composition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case musical composition = septet se.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.