Mid-month date in ancient Rome (4)
I believe the answer is:
ides
'mid-month date in ancient rome' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ides that I've seen before include "(In March) Caesar's time to beware" , "Caesar's fateful days" , "March day Caesar died" , "Eg March 15" , "Mid-month date in Roman calendar" .)