Caesar's fateful day (4)
I believe the answer is:
ides
'caesar's fateful day' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ides that I've seen before include "Period in month" , "(In March) Caesar's time to beware" , "date for 21 [ROMAN]?" , "Day in Roman months" , "Side of March when Caesar died" .)