French article containing one thing not true (3)
I believe the answer is:
lie
'thing not true' is the definition.
(lies are untrue)
'french article containing one' is the wordplay.
'french article' becomes 'le' (definite article in French).
'containing' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'le' enclosing 'i' is 'LIE'.
(Other definitions for lie that I've seen before include "Be prone - misrepresent the facts" , "Perversion of the truth" , "Stretch out" , "Pork pie" , "situation" .)