Make cryptic once out of French (6)
I believe the answer is:
encode
'make cryptic' is the definition.
(I've seen this before)
'once out of french' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'of french' becomes 'de' ('of' in French).
'once' anagrammed gives 'enco'.
'enco'+'de'='ENCODE'
(Other definitions for encode that I've seen before include "transform with substitutions?" , "Put (message) into ciphers" , "Encrypt" , "Convert into a cipher" , "Conceal meaning" .)