Composer Bernstein is sorrowful, though not entirely upset (7)
I believe the answer is:
rossini
'composer' is the definition.
(I've seen this before)
'bernstein is sorrowful though not entirely upset' is the wordplay.
'not entirely' says the answer is hidden in the clue.
'upset' says the letters should be written in reverse.
'inissor' is hidden in the letters of 'bernstein is sorrowful though'.
'inissor' in reverse letter order is 'ROSSINI'.
(Other definitions for rossini that I've seen before include "Gioacchino ---, It. composer" , "He composed operas" , "whose barber was in Seville" , "Italian composer, 1792-1868" , "Largo al factotum composer" .)