Written signs sanction barriers to trade by cartel (9)
I believe the answer is:
lettering
'written signs sanction' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'barriers to trade by cartel' is the wordplay.
I cannot really see how this works, but
'cartel' could be 'ring' (both can mean a criminal group) and 'ring' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for lettering that I've seen is " Inscribed decorative characters".)