Get ambassador to make tea? Language! (6)
I believe the answer is:
hebrew
'language' is the definition.
(I know that Hebrew is a modern language)
'get ambassador to make tea?' is the wordplay.
'get' means one lot of letters go next to another.
'ambassador' becomes 'he' (His or Her Excellency).
'to make tea?' becomes 'brew' (brew a pot of tea).
'he' next to 'brew' is 'HEBREW'.
(Other definitions for hebrew that I've seen before include "Israel's official language" , "Official language of Israel" , "Jewish language" , "Member of people" , "Tongue" .)