It follows a French one (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
'it follows a french' is the wordplay.
'follows' means one lot of letters go next to another.
'a french' becomes 'un' ('a' in French).
'it' after 'un' is 'UNIT'.
(Other definitions for unit that I've seen before include "A considerable number of soldiers" , "Item combining separate parts" , "detachment maybe" , "Any division of quantity accepted as a standard of measurement" , "Detachment of soldiers" .)