See about unusually bitter book (8)
I believe the answer is:
libretto
'book' is the definition.
(libretto is a kind of book)
'see about unusually bitter' is the wordplay.
'see' becomes 'lo' (archaic word meaning 'look!').
'about' means one lot of letters goes inside another.
'unusually' indicates an anagram.
'bitter' with letters rearranged gives 'ibrett'.
'lo' placed around 'ibrett' is 'LIBRETTO'.
(Other definitions for libretto that I've seen before include "Book of words of an opera" , "11 down book" , "component of opera" , "Operatic text" , "The words of an opera or musical play" .)