Boo's partner (4)
I believe the answer is:
hiss
'boo's partner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hiss that I've seen before include "Audio interference" , "Sharp sibilant sound" , "Sharp sound of disapproval" , "Urgent whisper" , "S-sound of disapproval" .)