Genuine regret after conclusion of Brexit (4)
I believe the answer is:
true
'genuine' is the definition.
('true' can be similar in meaning to 'genuine')
'regret after conclusion of brexit' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'after' says to put letters next to each other.
'conclusion of' suggests the final letters.
The final letter of 'brexit' is 't'.
'rue' after 't' is 'TRUE'.
(Other definitions for true that I've seen before include "Factually correct" , "not showing 12 [DISHONESTY]" , "Factually accurate" , "Real; loyal" , "Real; accurate" .)