The most common and most likely answer to this crossword clue is the 9 letter word GLISSANDI
I've seen this crossword clue in The Times
G
L
I
S
S
A
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I
More clues leading to the result GLISSANDI
- What involve sliding, as you will hear
- Slides sliding off, it separates
- Players may perform them, sliding as erratically
- Moves perhaps from A to B without stopping?
- Note series of coded signals I’d sent back
- Musical figurations, mostly smooth, encapsulating one’s polish
- What in music can be represented as sliding?
- Lad sings badly, including one rapidly executed series of notes
- German island is transported by musical effects
- “Slidings” achieved primarily when playing?
Buddy explains!

The straight part
We know that "Slides" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slides = glissandi.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 GLISSANDI mean?
GLISSANDI noun- Either a continuous sliding from one pitch to another ("true" glissando), or an incidental scale played while moving from one melodic note to another ("effective" glissando).
- A method of playing an electric guitar in which a metal bar is held at right angles across the strings and rapidly moved up and down, creating a smooth, lush sound.