Unusually brittle old book (8)
I believe the answer is:
libretto
'book' is the definition.
(libretto is a kind of book)
'unusually brittle old' is the wordplay.
'unusually' is an anagram indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'brittle' anagrammed gives 'librett'.
'librett'+'o'='LIBRETTO'
(Other definitions for libretto that I've seen before include "Operatic text" , "Book of words of an opera" , "11 down book" , "Book of words of a musical" , "Opera's words" .)