Cab heading off to the centre of Rome? That's taken for granted (5)
I believe the answer is:
'cab heading' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for axiom that I've seen before include "Self-evident statement" , "Dictum" , "Proposition assumed to be self-evident" , "Fundamental proposition" , "rule" .)