Awfully brittle old book (8)
I believe the answer is:
libretto
'book' is the definition.
(libretto is a kind of book)
'awfully brittle old' is the wordplay.
'awfully' is an anagram indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'brittle' is an anagram of 'librett'.
'librett'+'o'='LIBRETTO'
(Other definitions for libretto that I've seen before include "Operatic text" , "Words set to music, especially opera" , "Text set to music in opera" , "Book of words of an opera" , "textbook" .)