Formidable second book (8)
I believe the answer is:
hardback
'book' is the definition.
(I know that hardback is a type of book)
'formidable second' is the wordplay.
'formidable' becomes 'hard' (similar in meaning).
'second' becomes 'back' (to second a motion is to back it).
'hard'+'back'='HARDBACK'
(Other definitions for hardback that I've seen before include "It might be a novel!" , "Sort of book" , "Book with board covers" , "Penguins are rarely so" , "Style of book binding" .)