Once ran from French convert (6)
I believe the answer is:
encode
'convert' is the definition.
(I know that encode is a type of convert)
'once ran from french' is the wordplay.
'ran' indicates an anagram (letters have run around).
'from french' becomes 'de' (French for 'from').
'once' anagrammed gives 'enco'.
'enco'+'de'='ENCODE'
(Other definitions for encode that I've seen before include "Put (message) into ciphers" , "Convert into a cipher" , "transform with substitutions?" , "Put into secret writing" , "Encipher" .)