Possibly infinite series of language lessons (7)
I believe the answer is:
ageless
'possibly infinite' is the definition.
(never getting older)
'series of language lessons' is the wordplay.
'series of' indicates the answer is hidden within the clue (a series of letters from the clue).
'AGELESS' can be found hidden inside 'language lessons'.
(Other definitions for ageless that I've seen before include "Not appearing to grow older" , "Small amphibian" , "Sea-legs (anag.)" , "not getting on" , "not appearing to get old" .)