Sound of snake in bath is sinister (4)
I believe the answer is:
hiss
'sound of snake' is the definition.
(I've seen this before)
'in bath is sinister' is the wordplay.
'in' indicates the answer is hidden within the clue.
'HISS' can be found hidden inside 'bath is sinister'.
(Other definitions for hiss that I've seen before include "Sharp sound of disapproval" , "Make disapproving sound" , "Sound of escaping gas" , "S-sound of disapproval" , "Audio interference" .)