Roman calendar date (4)
I believe the answer is:
ides
'roman calendar date' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ides that I've seen before include "Mid-month date in Roman calendar" , "Caesar's fateful day" , "One da" , "Caesar's fateful days" , "Fateful day for Caesar" .)