Book is difficult to endorse (8)
I believe the answer is:
hardback
'book' is the definition.
(I know that hardback is a type of book)
'difficult to endorse' is the wordplay.
'difficult' becomes 'hard' (associated in meaning).
'to endorse' becomes 'back' ('back' can be a synonym of 'endorse').
'hard'+'back'='HARDBACK'
'is' acts as a link.
(Other definitions for hardback that I've seen before include "Book with stiff covers" , "Book with board covers" , "Sort of book" , "It might be a novel!" , "Penguins are rarely so" .)