A book that is unfinished doesn't come to an end (6)
I believe the answer is:
abides
'doesn't come to an end' is the definition.
(abide can mean to remain or continue)
'a book that is unfinished' is the wordplay.
'book' becomes 'b'.
'that is' becomes 'idest' ('that is' in Latin).
'unfinished' means to remove the last letter.
'idest' with its last letter removed is 'ides'.
'a'+'b'+'ides'='ABIDES'
(Other definitions for abides that I've seen before include "Dwells (archaic)" , "Lives or tolerates" , "Dwells or endures" , "Continues without fading or being lost, memory perhaps" , "Acts in accordance with rules" .)