Deeply regretted sounding impolite (4)
I believe the answer is:
rued
'deeply regretted' is the definition.
(I've seen this in another clue)
'sounding impolite' is the wordplay.
'sounding' shows a homophone (sound like).
'impolite' becomes 'rude' (similar in meaning).
'rude' is a homophone of 'RUED'.
(Other definitions for rued that I've seen before include "Was sorry" , "Bitterly regretted" , "was remorseful" , "Was contrite about" , "'Regretted, repented (4)'" .)