A French-Italian is one (4)
I believe the answer is:
unit
'one' is the definition.
('unit' can be a synonym of 'one')
'a french-italian' is the wordplay.
'a french' becomes 'un' ('a' in French**).
'italian' becomes 'it'.
'un'+'it'='UNIT'
'is' acts as a link.
(Other definitions for unit that I've seen before include "Standard for measuring" , "Module" , "Individual component" , "Single whole as part of a larger whole" , "Separate part" .)