Day in Roman month (4)
I believe the answer is:
ides
'day in roman month' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ides that I've seen before include "Mid-month day in the Roman calendar" , "day in Rome" , "Caesar's bad day in March" , "Caesar's fateful day" , "For example, March 15 in the Roman calendar" .)