Book finally reduced (5)
I believe the answer is:
atlas
'book' is the definition.
(atlas is a kind of book)
'finally reduced' is the wordplay.
'finally' becomes 'atlast' ('at last' can be a synonym of 'finally').
'reduced' means to remove the last letter.
'atlast' with its last letter taken off is 'ATLAS'.
(Other definitions for atlas that I've seen before include "The first cervical vertebra" , "Sort of book" , "I carry a huge weight" , "Album of maps" , "In Greek myth, a Titan made to support the heavens" .)