A hissed message, nothing less? (3)
I believe the answer is:
sos
'less?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sos that I've seen before include "urgent appeal" , "Appeal for help" , "Mayday call" , "Emergency message" , "request for aid" .)