Twice discovered source of the French language (4)
I believe the answer is:
urdu
'language' is the definition.
(Urdu is an example)
'twice discovered source of the french' is the wordplay.
'twice' becomes 'u' (I can't justify this - if you can you should believe this answer much more).
'discovered' says to take the centre (dis-covered - outer letters removed).
'source' becomes 'reservoir' (a reservoir is a supply or source).
'of the french' becomes 'du' ('of the' in French).
The centre of 'reservoir' is 'r'.
'u'+'r'+'du'='URDU'
Can you help me to learn more?
(Other definitions for urdu that I've seen before include "One of official languages of Pakistan" , "Pakistani tongue" , "Foreign language" , "Pakistani language" , "Language spoken by Moslems in Pakistan and India" .)