Hiss source (4)
I believe the answer is:
leak
'hiss source' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for leak that I've seen before include "Escape - make (confidential information) known" , "Spill secrets" , "Escape (of gas/liquid)" , "Seep; disclose" , "Unauthorised disclosure of confidential information" .)