Book for the opera (8)
I believe the answer is:
libretto
I believe this clue is a double definition.
'book' is the first definition.
(libretto is a kind of book)
'the opera' is the second definition.
(thesaurus)
'for' acts as a link.
(Other definitions for libretto that I've seen before include "Words set to music, especially opera" , "11 down book" , "Operatic book" , "Operatic text" , "component of opera" .)