a well-mannered fellow, extremely ideal, being “good breeding” as they once said - (8)
I believe the answer is:
threaten
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for threaten that I've seen before include "Cow" , "'Menace, intimidate (8)'" , "Put at risk, endanger" , "State an intention to harm" , "Look likely to" .)