Reference book is finally left unfinished (5)
I believe the answer is:
atlas
'reference book' is the definition.
(atlas is a kind of reference book)
'finally left unfinished' is the wordplay.
'finally' becomes 'atlast' (synonyms).
'left unfinished' means to remove the last letter.
'atlast' with its final letter taken away is 'ATLAS'.
'is' acts as a link.
(Other definitions for atlas that I've seen before include "Ballistic missile" , "Titan who bore the sky on his shoulders" , "Titan - map book" , "Asian satin" , "Album of maps" .)