World book finally abridged (5)
I believe the answer is:
Here is my best explanation:
'world book' is the definition.
'finally abridged' is the subsidiary indication.
'finally' becomes 'atlast' ('at last' can be a synonym of 'finally').
'abridged' means to remove the last letter.
'atlast' with its final letter removed is 'atlas'.
(Other definitions for atlas that I've seen before include "Album of maps", "A book of maps", "North African mountains - mythical Titan", "Collection of maps", "Chart book", "N African mountains - seemaps book", "N. African mountains; a Titan", "Book of geography", "Mythological Titan (involved with the world of maps?)" and "Its architectural name is telamon".)