The most common and most likely answer to this crossword clue is the 9 letter word VOLUNTARY
I've seen this crossword clue in The Guardian
V
O
L
U
N
T
A
R
Y
More clues leading to the result VOLUNTARY
Buddy explains!
The straight part
We know that "Famous trumpet piece/Not impressed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case famous trumpet piece/not impressed = voluntary.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 VOLUNTARY mean?
VOLUNTARY noun- A short piece of music, often having improvisation, played on a solo instrument.
- A volunteer.
- A supporter of voluntarism; a voluntarist.
- Done, given, or acting of one's own free will.
- Done by design or intention; intentional.
- Working or done without payment.
- Endowed with the power of willing.
- Of or relating to voluntarism.
- Voluntarily.