Cat book unfinished (3)
I believe the answer is:
tom
'cat' is the definition.
(I've seen this before)
'book unfinished' is the wordplay.
'book' becomes 'tome' (tome is a kind of book).
'unfinished' means to remove the last letter.
'tome' with its last letter removed is 'TOM'.
(Other definitions for tom that I've seen before include "Waits, perhaps" , "Boy running off with pig" , "Thumb" , "Hoyden" , "Boy - cat" .)