Roman mid-month date (4)
I believe the answer is:
ides
'roman mid-month date' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ides that I've seen before include "One da" , "March day Caesar died" , "Mid-month day in the Roman calendar" , "13th or 15th day of the month in Roman calendar" , "Side of March when Caesar died" .)