Real regret shown after time (4)
I believe the answer is:
true
'real' is the definition.
(synonyms)
'regret shown after time' is the wordplay.
I cannot really see how this works, but
'regret' could be 'rue' (synonyms) and 'rue' is present in the answer.
'time' could be 't' (abbreviation) and 't' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for true that I've seen before include "Genuine, loyal" , "Exact; faithful" , "In accordance with fact" , "Real, actual" , "to be believed?" .)