Genuine regret, after time (4)
I believe the answer is:
true
'genuine' is the definition.
(I know that genuine can be written as true)
'regret after time' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'after' means one lot of letters go next to another.
'time' becomes 't' (abbreviation).
'rue' after 't' is 'TRUE'.
(Other definitions for true that I've seen before include "Genuine, real" , "Honest - expression of agreement" , "Exact, factual" , "comfortable" , "to be believed?" .)