Slides, as with sliding rocks (9)
I believe the answer is:
glissandi
'slides' is the definition.
(sliding effects in music)
'as with sliding rocks' is the wordplay.
'with' means one lot of letters go next to another.
'rocks' is an anagram indicator.
'as' after 'sliding' is 'slidingas'.
'slidingas' anagrammed gives 'GLISSANDI'.
(Other definitions for glissandi that I've seen before include "Series of rapidly ascending or descending musical notes" , "showing smooth actions" , "Players may perform them" , "playing, while sliding up and down the passages" , "Musical terms" .)