An air of sadness (5)
I believe the answer is:
dirge
This clue could be a double definition.
'an air' is the first definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe there's an association between them I don't understand?
'sadness' is the second definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for dirge that I've seen before include "Dreary song" , "Elegy" , "Lamentation" , "feature of funeral" , "Funeral lament" .)