Bad day for Caesar (4)
I believe the answer is:
ides
'bad day for caesar' is the definition.
(Julius Caesar was killed on the Ides of March)
This is all the clue.
(Other definitions for ides that I've seen before include "Julius Caesar was killed during the . . . . of March" , "For example, March 15 in the Roman calendar" , "March day Caesar died" , "Day in Roman months" , "Fatal March date or Julius Caesar" .)