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