French are supporting one date, a significant date (4)
I believe the answer is:
ides
'a significant date' is the definition.
'ides' can be an answer for 'date' (I've seen this before). I am unsure of the 'a significant' bit.
'french are supporting one date' is the wordplay.
'french are' becomes 'es' ('are' in French).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'one' becomes 'i' (Roman numeral).
'date' becomes 'd' (common abbreviation eg in DOB, date of birth).
'i'+'d'='id'
'es' after 'id' is 'IDES'.
Can you help me to learn more?
(Other definitions for ides that I've seen before include "ill-fated time" , "mid-month for the Romans" , "March 15; dies (anag.)" , "Orfes" , "13th or 15th day of the month in Roman calendar" .)