Regretted sounding ill-mannered (4)
I believe the answer is:
rued
'regretted' is the definition.
('rue' can be a synonym of 'regret')
'sounding ill-mannered' is the wordplay.
'sounding' indicates a 'sounds like' (homophone) clue.
'ill-mannered' becomes 'rude' (synonyms).
'rude' sounds like 'RUED'.
(Other definitions for rued that I've seen before include "Was sorry" , "was remorseful" , "'Regretted, repented (4)'" , "Was contrite about" , "Bitterly regretted" .)