People no longer speak it, but Upman can (5)
I believe the answer is:
latin
'people no longer speak it' is the definition.
I can't judge whether this defines the answer.
'upman can' is the wordplay.
'upman' becomes 'la' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'can' becomes 'tin' (synonyms).
'la'+'tin'='LATIN'
'but' is the link.
Can you help me to learn more?
(Other definitions for latin that I've seen before include "Hispanic, for example" , "Shakespeare's was small, according to Jonson" , "Classical language" , "Ancestor of French, Romanian" , "language class" .)