Regretted vulgar broadcast (4)
I believe the answer is:
rued
'regretted' is the definition.
('rue' can be a synonym of 'regret')
'vulgar broadcast' is the wordplay.
'vulgar' becomes 'rude' (vulgar can mean rude or gross).
'broadcast' indicates a 'sounds like' (homophone) clue (how the word might sound on the radio or TV).
'rude' sounds like 'RUED'.
(Other definitions for rued that I've seen before include "Bitterly regretted" , "'Regretted, repented (4)'" , "Was contrite about" , "was remorseful" , "Was sorry" .)