Signal back, too (3)
I believe the answer is:
sos
This might be a double definition.
'signal' is the first definition.
(SOS is an example)
'back too' is the second definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
Can you help me to learn more?
(Other definitions for sos that I've seen before include "Mayday call" , "request for aid" , "Distress call" , "Help code in 13 Across" , "Morse Code for help" .)