Reference book finally abridged (5)
I believe the answer is:
atlas
'reference book' is the definition.
(atlas is a kind of reference book)
'finally abridged' is the wordplay.
'finally' becomes 'at last' (synonyms).
'abridged' means to remove the last letter.
'atlast' with its last letter removed is 'ATLAS'.
(Other definitions for atlas that I've seen before include "The first cervical vertebra" , "Mythological Titan (involved with the world of maps?)" , "collected world records?" , "Sky-toting Titan" , "Sort of book" .)