Endlessly mouth impenetrable old language (6)
I believe the answer is:
gothic
'old language' is the definition.
(I've seen this before)
'endlessly mouth impenetrable' is the wordplay.
'endlessly' means to remove the last letter.
'mouth' becomes 'gob' (informal term for the mouth).
'impenetrable' becomes 'thic' (I can't justify this - if you can you should believe this answer much more).
'gob' with its last letter removed is 'go'.
'go'+'thic'='GOTHIC'
Can you help me to learn more?
(Other definitions for gothic that I've seen before include "Style between Romanesque and Classical" , "Architectural style of the Middle Ages" , "Architectural or literary style" , "Style of architecture and gloomy books" , "as some Germans were" .)