Polite one, an ordinary person (8)
I believe the answer is:
civilian
'ordinary person' is the definition.
'civilian' can be an answer for 'person' (civilian is a kind of person). I am not certain of the 'ordinary' bit.
'polite one an' is the wordplay.
'polite' becomes 'civil' (synonyms).
'one' becomes 'I' (Roman numeral).
'civil'+'i'+'an'='CIVILIAN'
Can you help me to learn more?
(Other definitions for civilian that I've seen before include "Member of a country" , "Forces apart any person" , "One is not a member of the police or armed forces" , "Layman found in a civil way" , "Nonmilitary person" .)