An indication of distress (3)
I believe the answer is:
sos
This clue could be a double definition.
'an indication' is the first definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe there's a link between them I don't understand?
'distress' is the second definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'of' is the link.
Can you help me to learn more?
(Other definitions for sos that I've seen before include "Morse Code for help" , "Call for help" , "urgent appeal" , "request for aid" , "Help code in 13 Across" .)