One's neither civil nor polite, but rude or foul! (4-6,4)

'foul' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'one's neither civil nor polite but rude or' is the wordplay.
I cannot really see how this works, but
'or' is within the answer.
'one' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
This explanation may well be incorrect...