Book that's 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' (synonyms).
'to endorse?' becomes 'back' (synonyms).
'hard'+'back'='HARDBACK'
'that's' is the link.
(Other definitions for hardback that I've seen before include "Book with board covers" , "It might be a novel!" , "Penguins are rarely so" , "Book with stiff covers" , "Sort of book" .)