Show disapproval of incomplete tome (3)
I believe the answer is:
boo
'show disapproval of' is the definition.
(as in a crowd booing)
'incomplete tome' is the wordplay.
'incomplete' means to remove the last letter.
'tome' becomes 'book' (I've seen this in another clue).
'book' with its final letter taken away is 'BOO'.
(Other definitions for boo that I've seen before include "A shocking exclamation!" , "Exclamation to startle" , "Shout of dislike" , "Voice disapproval" , "Shout in disgust" .)