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