Article about friend — all in French — becoming monster (5)
I believe the answer is:
lamia
'all in french becoming monster' is the definition.
'lamia' can be an answer for 'monster' (thesaurus). I am not certain of the 'all in french becoming' bit.
'article about friend' is the wordplay.
'article' becomes 'la' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'friend' becomes 'ami' (I've seen this before).
'la' enclosing 'ami' is 'LAMIA'.
(Other definitions for lamia that I've seen before include "blood-sucking monster" , "vampire" , "Serpent-witch's" .)