Book challenging champion (8)
I believe the answer is:
hardback
'book' is the definition.
(I know that hardback is a type of book)
'challenging champion' is the wordplay.
'challenging' becomes 'hard' (similar in meaning).
'champion' becomes 'back' (both can mean to endorse or support).
'hard'+'back'='HARDBACK'
(Other definitions for hardback that I've seen before include "Sort of book" , "Durable version of book" , "Penguins are rarely so" , "Book with stiff covers" , "It might be a novel!" .)