Black ducks show disapproval (3)
I believe the answer is:
boo
'show disapproval' is the definition.
(as in a crowd booing)
'black ducks' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'ducks' becomes 'OO' (two O's - duck means zero in cricket).
'b'+'oo'='BOO'
(Other definitions for boo that I've seen before include "Negative reaction" , "I don't like that!" , "Cry of disapproval as at villain" , "Show displeasure orally" , "Surprising outburst" .)