Lack of interest in Rossini unnecessarily picked up (5)
I believe the answer is:
ennui
'lack of interest' is the definition.
(I've seen this before)
'in rossini unnecessarily picked up' is the wordplay.
'in' says the answer is hidden in the clue.
'picked up' says the letters should be written backwards.
'iunne' is hidden in the letters of 'rossini unnecessarily'.
'iunne' in reverse letter order is 'ENNUI'.
(Other definitions for ennui that I've seen before include "Boredom or discontent" , "bore" , "Lower interest rate" , "Listless boredom" , "Boredom, dullness" .)