Regretted being rumbled, disheartened (4)
I believe the answer is:
rued
'regretted' is the definition.
('rue' can be a synonym of 'regret')
'rumbled disheartened' is the wordplay.
'disheartened' says to hollow out the word (remove centre letters) (the middle or heart is removed).
'rumbled' with its middle taken out is 'RUED'.
'being' is the link.
(Other definitions for rued that I've seen before include "was remorseful" , "'Regretted, repented (4)'" , "Was sorry" , "Was contrite about" , "Bitterly regretted" .)