Book finally shortened (5)
I believe the answer is:
atlas
'book' is the definition.
(atlas is a kind of book)
'finally shortened' is the wordplay.
'finally' becomes 'atlast' (synonyms).
'shortened' means to remove the last letter.
'atlast' with its last letter removed is 'ATLAS'.
(Other definitions for atlas that I've seen before include "In Greek myth, a Titan made to support the heavens" , "The first cervical vertebra" , "world record?" , "One greatly burdened" , "Chart book" .)