Official document the French sent up (6)
I believe the answer is:
formal
'official' is the definition.
(similar in meaning)
'document the french sent up' is the wordplay.
'document' becomes 'form' (form is a kind of document).
'the french' becomes 'la'.
'sent up' says the letters should be written in reverse.
'la' written backwards gives 'al'.
'form'+'al'='FORMAL'
(Other definitions for formal that I've seen before include "black tie" , "Punctilious" , "Ceremonious or official" , "Ceremonial or official" , "Done in accordance with etiquette" .)