Reference book finally reduced (5)
I believe the answer is:
atlas
'reference book' is the definition.
(atlas is a kind of reference 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 removed is 'ATLAS'.
(Other definitions for atlas that I've seen before include "Titan who held up the sky" , "Legendary weightlifter" , "Set of charts" , "Size of paper once" , "Cartographer's work" .)