Cry of derision (7)
(Other definitions for boo that I've seen before include "Surprising outburst" , "Show displeasure orally" , "Exclamation to startle" , "Voice disapproval" , "Shout of disapproval, maybe at ref" .)
(Other definitions for catcall that I've seen before include "Audience's expression of disapproval" , "disapproving expression" , "Boo" , "taunt from the audience" , "Audibly disapprove" .)