Nervous girl reported plain language in America (8)
I believe the answer is:
cheyenne
'plain language in america' is the definition.
(Native American language - language used on plains)
I don't understand how the rest of the clue works.
(Other definitions for cheyenne that I've seen before include "American Indian" , "North American native" , "tongue" , "US state capital" , "Native American of the western Plains" .)