A dashed dotty broadcast appeal? (3)

I believe the answer is:

sos

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'appeal?' is the definition.
(I have seen 'urgent appeal' mean 'sos' so perhaps 'appeal' could also mean 'sos')

I don't 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 "request for aid" , "urgent appeal" , "Mayday call" , "Coded distress signal" , "Help with dots and dashes" .)

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