The most common and most likely answer to this crossword clue is the 10 letter word SOUSAPHONE
I've seen this crossword clue in The Times
S
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Buddy explains!
The straight part
We know that "brass" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case brass = sousaphone.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 SOUSAPHONE mean?
SOUSAPHONE noun- A valved brass instrument with the same length as a tuba, but shaped differently so that the bell is above the head, that the valves are situated directly in front of the musical instruments and a few inches above the waist, and that most of the weight rests on one shoulder.