Obscure book — promotional copy? (5)
I believe the answer is:
blurb
'promotional copy?' is the definition.
(copy or text promoting a book)
'obscure book' is the wordplay.
'obscure' becomes 'blur' (both can mean to vague or indistinct).
'book' becomes 'b' (abbreviation for book).
'blur'+'b'='BLURB'
(Other definitions for blurb that I've seen before include "Book endorsement" , "Advertising spiel" , "printed on jacket" , "Publisher's write-up" , "Publisher's promotional text" .)