Finally doesn't finish book (5)
I believe the answer is:
atlas
'book' is the definition.
(atlas is a kind of book)
'finally doesn't finish' is the wordplay.
'finally' becomes 'at last' (synonyms).
'doesn't finish' 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 "N African mountains - seemaps book" , "Titan - map book" , "Supporter of the classics" , "Reference work" , "Holder of world" .)