Plain "show one in" before... (7)
I believe the answer is:
prairie
'plain' is the definition.
(type of plain)
'show one in before' is the wordplay.
'show' becomes 'air' (both can mean broadcast).
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'before' becomes 'pre' (prefix meaning 'before').
'air'+'i'='airi'
'airi' going within 'pre' is 'PRAIRIE'.
(Other definitions for prairie that I've seen before include "North American grassland" , "Large tract of grassland in N America" , "Open grassland of North America" , "Large grassland region" , "American plain" .)