One of the French days in ancient Rome (4)
I believe the answer is:
ides
'days in ancient rome' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'one of the french' is the wordplay.
'one' becomes 'i' (Roman numeral).
'of the french' becomes 'des' ('of the' in French).
'i'+'des'='IDES'
Can you help me to learn more?
(Other definitions for ides that I've seen before include "13th or 15th day of the month in Roman calendar" , "Fateful day for Caesar" , "Julius Caesar was told to beware the . . . . of March" , "Mid-month day in the Roman calendar" , "Fatal March date or Julius Caesar" .)