Time to regret being straight (4)
I believe the answer is:
true
'straight' is the definition.
('true' can be a synonym of 'straight')
'time to regret' is the wordplay.
'time' becomes 't'.
'to regret' becomes 'rue' ('rue' can be a synonym of 'regret').
't'+'rue'='TRUE'
'being' acts as a link.
(Other definitions for true that I've seen before include "Straight or correct" , "Dependable -- loyal" , "Genuine, real" , "Honest and straight" , "Veritable" .)