Genuine regret at the end of it (4)
I believe the answer is:
true
'genuine' is the definition.
('true' can be similar in meaning to 'genuine')
'regret at the end of it' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'at the end of' is a charade indicator (letters next to each other).
'it' becomes 't' (abbreviation. e.g. in 'tis).
'rue' put after 't' is 'TRUE'.
(Other definitions for true that I've seen before include "Factually correct" , "Straight or loyal" , "comfortable" , "Agreeing with fact or reality" , "Right; genuine" .)