Extent of French language ultimately wanting (6)
I believe the answer is:
degree
'extent' is the definition.
(I've seen this before)
'of french language ultimately wanting' is the wordplay.
'of french' becomes 'de' ('of' in French).
'language' becomes 'Greek' (Greek is an example).
'ultimately wanting' means to remove the last letter (wanting can mean lacking).
'greek' with its last letter taken away is 'gree'.
'de'+'gree'='DEGREE'
(Other definitions for degree that I've seen before include "Comparative amount" , "Gradation on a scale" , "displayed by small elevated circle" , "Letters" , "Angular unit; grade" .)