Hardy's fury about books reflecting decline (8)
I believe the answer is:
intrepid
'hardy's' is the definition.
(thesaurus)
'fury about books reflecting decline' is the wordplay.
'fury' becomes 'ire' (I've seen this before).
'about' means one lot of letters goes inside another.
'books' becomes 'nt' (New Testament).
'reflecting' says the letters should be written backwards.
'decline' becomes 'dip' (dipping is a kind of declining**).
'dip' back-to-front is 'pid'.
'ire' enclosing 'nt' is 'intre'.
'intre'+'pid'='INTREPID'
(Other definitions for intrepid that I've seen before include "Audacious" , "Brave and fearless" , "stout" , "Fearless, adventurous" , "Bold" .)