Regret a lot of painful argument (6)
I believe the answer is:
sorrow
'regret' is the definition.
(I've seen this in another clue)
'a lot of painful argument' is the wordplay.
'a lot of' means to remove the last letter (a lot of but not all of the word).
'painful' becomes 'sore' (similar in meaning).
'argument' becomes 'row' (to row is to argue).
'sore' with its final letter taken away is 'sor'.
'sor'+'row'='SORROW'
(Other definitions for sorrow that I've seen before include "Deep sadness" , "Shakespeare: ''Parting is such sweet . . . . . .'' (6)" , "Deep distress" , "Sadness, feeling of regret" , "Sadness; one magpie for it" .)