Obscure description of book that's unfinished (4)
I believe the answer is:
blur
'obscure' is the definition.
(both can mean to vague or indistinct)
'description of book that's unfinished' is the wordplay.
'description of book' becomes 'blurb' (description on the back of a book).
'that's unfinished' means to remove the last letter.
'blurb' with its last letter taken off is 'BLUR'.
(Other definitions for blur that I've seen before include "Unfocused image" , "Smudge - Britpop band" , "Out-of-focus image" , "Haze - obscure" , "Out-of-focus sight" .)