Smear something on back of book obscuring ending (4)
I believe the answer is:
blur
'smear' is the definition.
(I know that smear can be written as blur)
'something on back of book obscuring ending' is the wordplay.
'something on back of book' becomes 'blurb' (promotional copy on the back of a book).
'obscuring ending' means to remove the last letter.
'blurb' with its last letter removed is 'BLUR'.
(Other definitions for blur that I've seen before include "fade" , "Make hazy" , "Smudge - Britpop band" , "murky spot" , "Unclear image, of a pop band" .)