Miss appointed number one in French Department (12)
I believe the answer is:
mademoiselle
'miss' is the definition.
(thesaurus)
'appointed number one in french department' is the wordplay.
'appointed' becomes 'made' (making is a kind of appointing).
'number one' becomes 'I' (Roman numeral).
'in' means one lot of letters goes inside another.
'french department' becomes 'moselle' (I've seen this before).
'i' put into 'moselle' is 'moiselle'.
'made'+'moiselle'='MADEMOISELLE'
(Other definitions for mademoiselle that I've seen before include "Foreign woman" , "French miss" , "Parisian girl" , "Young lady" , "Paris girl" .)