Publicity broadcast that is plain (7)
I believe the answer is:
prairie
'plain' is the definition.
(type of plain)
'publicity broadcast that is' is the wordplay.
'publicity' becomes 'pr'.
'broadcast' becomes 'air' (synonyms).
'that is' becomes 'i.e.' (short for Latin 'id est', 'that is').
'pr'+'air'+'ie'='PRAIRIE'
(Other definitions for prairie that I've seen before include "Continental grassland of N America" , "North American grassland" , "Large grassland region" , "Extensive US grassland" , "Tract of (US) grassland" .)