"Et tu" day (4)
I believe the answer is:
ides
'et tu day' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ides that I've seen before include "Eg March 15" , "Julius Caesar was told to beware the . . . . of March" , "Period in month" , "Caesar was warned to beware the ones of March" , "day in Rome" .)