Maritime distress signal (6)
(Other definitions for sos that I've seen before include "request for aid" , "Help code in 13 Across" , "Coded distress signal" , "urgent appeal" , "Call for help" .)
(Other definitions for mayday that I've seen before include "Cry of distress" , "Request for help" , "I need help" , "Help urgently needed" , "International radio distress signal" .)