Still hissing? (6)
I believe the answer is:
static
I believe this clue is a double definition.
'still' is the first definition.
('static' can be a synonym of 'still')
'hissing?' is the second definition.
(thesaurus)
(Other definitions for static that I've seen before include "Still - type of electricity" , "Unmoving, unchanging" , "stable" , "Its cat (anag)" , "Inert, motionless" .)