The most common and most likely answer to this crossword clue is the 9 letter word EUPHONIUM
I've seen this crossword clue in The Guardian
E
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More clues leading to the result EUPHONIUM
Buddy explains!
The straight part
We know that "blow it!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case blow it! = euphonium.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 EUPHONIUM mean?
EUPHONIUM noun- A valved brass instrument, a sax horn, the tenor of the tuba family of instruments, having the appearance very similar to that of a tuba. It is similar to and often used instead of a "Baritone" horn. (A true Baritone has a cylindrical tubing, while the Euphonium tubing is conical, though they both cover the same range of tones. This relationship is also between the Trumpet [cylindrical] and Cornet [conical] respectively.)