Mayday! (4)
(Other definitions for help that I've seen before include "Succour" , "Take what you want" , "Lend a hand" , "Support" , "Beatles' album; assist" .)
'mayday' is the definition.
(I've seen this in other clues)
This is all the clue.
(Other definitions for sos that I've seen before include "Call for help" , "International distress code" , "plea" , "Mayday call" , "request for aid" .)