Genuine regret after Truss's first (4)
I believe the answer is:
true
'genuine' is the definition.
(I know that genuine can be written as true)
'regret after truss's first' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'after' says to put letters next to each other.
'truss's first' becomes 't' (1st letter of 'truss').
'rue' after 't' is 'TRUE'.
(Other definitions for true that I've seen before include "Supported by the facts" , "Consistent with fact" , "Corrected" , "Bona fide" , "Honest and straight" .)