The most common and most likely answer to this crossword clue is the 9 letter word HALF-STEP
I've seen this crossword clue in Financial Times
H
A
L
F
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S
T
E
P
Buddy explains!
The straight part
We know that "Broadway musical interval" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case broadway musical interval = half-step.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 HALF-STEP mean?
HALF-STEP noun- An interval of which there are seven in a fifth or twelve in an octave, consisting of two tones whose frequency ratio is about 1.059 or 18/17.