Piece of libretto, shocking drivel (4)
I believe the answer is:
tosh
'drivel' is the definition.
(I've seen this before)
'piece of libretto shocking' is the wordplay.
'piece of' says the answer is hidden in the clue.
'TOSH' can be found hidden inside 'libretto shocking'.
(Other definitions for tosh that I've seen before include "[BLAH]" , "moonshine" , "garbage'" , "Nonsense (colloq.)" , "Cobblers" .)