"Et tu, Brute?" day (4)
I believe the answer is:
ides
'et tu brute? day' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ides that I've seen before include "Julius Caesar died in these March days" , "Caesar's fateful day" , "Period in month" , "day in Rome" , "Julius Caesar was told to beware the . . . . of March" .)