Reference book eventually cut short (5)
I believe the answer is:
atlas
'reference book' is the definition.
(atlas is a kind of reference book)
'eventually cut short' is the wordplay.
'eventually' becomes 'at last' (at long last).
'cut short' 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 "Giant" , "North African mountains - mythical Titan" , "Chart book" , "Segment of spine" , "I carry a huge weight" .)