Language used by French philosopher is out of line (6)
I believe the answer is:
patois
'language used by french' is the definition.
(term for a non-standard dialect of French)
'philosopher is out of line' is the wordplay.
'philosopher' becomes 'plato' (Plato is an example).
'out of' indicates named letters should be taken away.
'line' becomes 'l'.
'plato'+'is'='platois'
'platois' with 'l' taken out is 'PATOIS'.
(Other definitions for patois that I've seen before include "A regional dialect of a language" , "Local dialect heard in patios" , "speech" , "Spoken regional dialect" , "Kind of dialect, from French" .)