Boy's big book left unfinished (3)
I believe the answer is:
tom
'boy's' is the definition.
'big book left unfinished' is the wordplay.
'big book' becomes 'tome' (I've seen this before).
'left unfinished' means to remove the last letter.
'tome' with its final letter taken away is 'TOM'.
(Other definitions for tom that I've seen before include "- Lehrer, US humorist" , "Edgar's Poor disguise (Lear)" , "Thumb" , "Pig-stealing piper's son" , "Queen's mate" .)