Said to have regretted being impolite (4)
I believe the answer is:
rude
'impolite' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for rude that I've seen before include "Impolite - simple" , "Offensively impolite" , "'Bad-mannered, discourteous (4)'" , "Unpleasant" , "Base" .)