The French girl is perfectly proper (5)
I believe the answer is:
legal
'perfectly proper' is the definition.
'legal' can be an answer for 'proper' (thesaurus). I'm not sure about the 'perfectly' bit.
'the french girl' is the wordplay.
'the french' becomes 'le'.
'girl' becomes 'gal' ('gal' can be a synonym of 'girl'**).
'le'+'gal'='LEGAL'
'is' is the link.
Can you help me to learn more?
(Other definitions for legal that I've seen before include "Within bounds of law" , "In accordance with statute" , "not breaking the law" , "OK as an immigrant?" , "Referring to the law" .)