Public relations introduce theme that is plain (7)
I believe the answer is:
prairie
'plain' is the definition.
(type of plain)
'public relations introduce theme that is' is the wordplay.
'public relations' becomes 'PR' (abbreviation).
'introduce theme' becomes 'air' (I can't justify this - if you can you should give a lot more credence to this answer).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'pr'+'air'+'ie'='PRAIRIE'
Can you help me to learn more?
(Other definitions for prairie that I've seen before include "Open grassland in the USA" , "Treeless grassy plain, in the old West perhaps" , "Extensive US grassland" , "Tract of (US) grassland" , "Schooners once crossed it" .)