Such figures are not Roman (6)
I believe the answer is:
arabic
'roman' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'such figures are' is the wordplay.
I cannot really see how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found within the answer.
The remaining letters 'rabic' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'not' is the link.
Can you help me to learn more?
(Other definitions for arabic that I've seen before include "foreign language" , "Correspond; last word of Othello" , "Language (of numbers)" , "There's a CIA bar on this language, oddly" , "Major language of Middle East" .)