A message mostly hissed? (3)

I believe the answer is:

sos

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'a message' is the definition.

I cannot understand how the rest of the clue works.

(Other definitions for sos that I've seen before include "urgent appeal" , "Appeal for help" , "request for aid" , "Emergency message" , "Help code in 13 Across" .)

I've seen this clue in the Evening Standard.

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