Look up in native American language (6)
I believe the answer is:
creole
'language' is the definition.
(I know that creole is a type of natural language)
'look up in native american' is the wordplay.
'look' becomes 'lo' (archaic exclamation meaning 'look!').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' is an insertion indicator.
'native american' becomes 'cree' (an indigenous people of North America).
'lo' back-to-front is 'ol'.
'ol' inserted into 'cree' is 'CREOLE'.
(Other definitions for creole that I've seen before include "People in Louisiana with French ancestors" , "spicy cookery" , "One of mixed descent" , "Descendant of French settlers as in New Orleans" , "White descendant of French ancestors as in New Orleans" .)