Bell's unfinished book (3)
I believe the answer is:
tom
'bell's' is the definition.
(I've seen this before)
'unfinished book' is the wordplay.
'unfinished' means to remove the last letter.
'book' becomes 'tome' (a tome is a book).
'tome' with its final letter taken off is 'TOM'.
(Other definitions for tom that I've seen before include "Waits, perhaps" , "Boy running off with pig" , "Boy - cat" , "Male feline" , "Cruise" .)