One of French, most celebrated in Cannes, primarily as visionary (10)
I believe the answer is:
Here is my best explanation:
'visionary' is the definition.
(I've seen this before)
'one of french most celebrated in cannes primarily' is the wordplay.
'one' becomes 'i' (Roman numeral).
'of french' becomes 'de'.
'most celebrated' becomes 'alist' (the A-list are top celebrities).
'primarily' suggests taking the first letters.
The first letters of 'in cannes' is 'ic'.
'as' acts as a link.
(Other definitions for idealistic that I've seen before include "striving for perfection", "Impracticable", "High-minded - italicised (anag)", "Seeking the best", "Naively impractical", "Utopian", "Visionary" and "Striving for perfection".)