Precise time to regret (4)
I believe the answer is:
true
'precise' is the definition.
(thesaurus)
'time to regret' is the wordplay.
'time' becomes 't' (abbreviation).
'to regret' becomes 'rue' (synonyms).
't'+'rue'='TRUE'
(Other definitions for true that I've seen before include "Exact; faithful" , "Accurate, factual" , "Honest and straight" , "Straight or loyal" , "Honest - expression of agreement" .)