Challenging second book (8)
I believe the answer is:
hardback
'book' is the definition.
(hardback is a kind of book)
'challenging second' is the wordplay.
'challenging' becomes 'hard' ('hard' can be similar in meaning to 'challenging').
'second' becomes 'back' (to second a motion is to back it).
'hard'+'back'='HARDBACK'
(Other definitions for hardback that I've seen before include "Book with stiff covers" , "Book with board covers" , "It might be a novel!" , "Penguins are rarely so" , "Durable version of book" .)