Genuine regret after act's conclusion (4)
I believe the answer is:
true
'genuine' is the definition.
(I know that genuine can be written as true)
'regret after act's conclusion' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'after' says to put letters next to each other.
'conclusion' indicates one should take the final letters.
The last letter of 'act' is 't'.
'rue' put after 't' is 'TRUE'.
(Other definitions for true that I've seen before include "Straight or correct" , "Straight or loyal" , "Consistent with fact" , "In accordance with fact" , "Dependable -- loyal" .)