Public relations broadcast that is plain (7)
I believe the answer is:
prairie
'plain' is the definition.
(thesaurus)
'public relations broadcast that is' is the wordplay.
'public relations' becomes 'pr'.
'broadcast' becomes 'air' ('air' can be a synonym of 'broadcast').
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'pr'+'air'+'ie'='PRAIRIE'
(Other definitions for prairie that I've seen before include "Tract of (US) grassland" , "Treeless grassy plain, in the old West perhaps" , "Continental grassland of N America" , "Extensive US grassland" , "Open grassland in the USA" .)