Cry of disapproval (3)
'cry of disapproval' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for boo that I've seen before include "Surprising exclamation" , "Frightening word (for a goose?)" , "Cry of disapproval as at villain" , "I'm unimpressed" , "Startling utterance" .)
(Other definitions for catcall that I've seen before include "Boo" , "Noisy expression of disapproval" , "Derisive call" , "1express disapproval" , "Shout of derision" .)