Right time to regret (4)
I believe the answer is:
true
'right' is the definition.
('true' is associated in meaning with 'right')
'time to regret' is the wordplay.
'time' becomes 't'.
'to regret' becomes 'rue' ('rue' can be a synonym of 'regret').
't'+'rue'='TRUE'
(Other definitions for true that I've seen before include "A false alternative?" , "Loyal; authentic" , "Real; loyal" , "Right; genuine" , "Agreeing with fact or reality" .)