Regretted sounding most uncivil (4)
I believe the answer is:
rued
'regretted' is the definition.
('rue' can be a synonym of 'regret')
'sounding most uncivil' is the wordplay.
I cannot really see how this works, but
'sounding' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
The remaining letters 'rud' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rued that I've seen before include "was remorseful" , "Was sorry" , "'Regretted, repented (4)'" , "Bitterly regretted" , "Was contrite about" .)