Exude an air of sadness (4)
I believe the answer is:
sigh
'air of sadness' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sigh that I've seen before include "resigned response" , "Sound of despondency" , "Yearn" , "Express relief or sorrow" , "Sound of regret" .)