Book difficult to endorse (8)
I believe the answer is:
hardback
'book' is the definition.
(hardback is a kind of book)
'difficult to endorse' is the wordplay.
'difficult' becomes 'hard' ('hard' can be a synonym of 'difficult').
'to endorse' becomes 'back' ('back' can be a synonym of 'endorse').
'hard'+'back'='HARDBACK'
(Other definitions for hardback that I've seen before include "Style of book binding" , "Penguins are rarely so" , "Book with board covers" , "It might be a novel!" , "Durable version of book" .)