Soft cases used by opera violinists (7)
I believe the answer is:
ravioli
'soft cases' is the definition.
(cases or envelopes of pasta)
'used by opera violinists' is the wordplay.
'used by' indicates the answer is hidden within the clue.
'RAVIOLI' is hidden within 'opera violinists'.
(Other definitions for ravioli that I've seen before include "you might have at La Scala?" , "stuffed envelopes to eat" , "Italian dish (1)" , "Italian pillow-shaped pasta" , "Filled envelopes of course" .)