Numerically different from Arabic (5)
I believe the answer is:
roman
'arabic' is the definition.
The answer and definition 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 how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for roman that I've seen before include "(Of a nose) high-bridged" , "A novel" , "Audrey Hepburn film (time off in Italy)" , "Locking device" , "Plain upright type" .)