Time to regret being faithful (4)
I believe the answer is:
true
'faithful' is the definition.
(associated in meaning)
'time to regret' is the wordplay.
'time' becomes 't' (abbreviation).
'to regret' becomes 'rue' (to rue something is to regret it).
't'+'rue'='TRUE'
'being' acts as a link.
(Other definitions for true that I've seen before include "Real; accurate" , "Genuine, actual" , "Constant" , "comfortable" , "Bona fide" .)