Principle unchanged, on reflection (5)
I believe the answer is:
tenet
This clue might be a double definition.
'principle' is the first definition.
(a tenet is a principle or value)
'unchanged on reflection' is the second definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps there's a link between them I don't understand?
Can you help me to learn more?
(Other definitions for tenet that I've seen before include "Adiaphoron" , "Canon" , "Belief held as true" , "Article of faith; doctrine" , "Conviction" .)