It's polite not to use Arabic numerals (5)
I believe the answer is:
civil
'it's polite' is the definition.
(synonyms)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for civil that I've seen before include "Courteous, polite" , "Nonviolent refusal to obey laws" , "(Of law) non-criminal" , "Relating to citizens" , "Mannerly" .)