Regretted, say, being impolite (4)
I believe the answer is:
rude
'impolite' is the definition.
(similar in meaning)
'regretted say' is the wordplay.
'regretted' becomes 'rued' ('rue' can be a synonym of 'regret').
'say' indicates a 'sounds like' (homophone) clue.
'rued' sounds like 'RUDE'.
'being' is the link.
(Other definitions for rude that I've seen before include "Impolite or coarse" , "Impolite - simple" , "Impolite; unpolished" , "Coarse or discourteous" , "Uncultivated" .)