Language of half the universe (4)
I believe the answer is:
erse
'language' is the definition.
(historical term for Scottish Gaelic)
'half the universe' is the wordplay.
'half the' suggests halving the letters.
'universe' cut in half is 'ERSE'.
'of' acts as a link.
(Other definitions for erse that I've seen before include "Old Scots or Irish Gaelic" , "Possibly the language" , "Old name for Celtic languages" , "Celtic tongue" , "Irish Gaelic language" .)