Public relations broadcast that is plain for Americans (7)
I believe the answer is:
'plain for americans' is the definition.
(prairie can mean an American plain)
'public relations broadcast that is' is the wordplay.
'public relations' becomes 'PR' (abbreviation).
'broadcast' becomes 'air' (to air on TV).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
(Other definitions for prairie that I've seen before include "Large open area of grassland in the US" , "Treeless grassy plain, in the old West perhaps" , "American plain" , "Extensive US grassland" , "Large treeless tract of grassland" .)