Plain expression that is concluding publicity (7)
I believe the answer is:
prairie
'plain' is the definition.
(thesaurus)
'expression that is concluding publicity' is the wordplay.
'expression' becomes 'air' (I've seen this before).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'concluding' means one lot of letters go next to another.
'publicity' becomes 'pr'.
'air'+'ie'='airie'
'airie' put after 'pr' is 'PRAIRIE'.
(Other definitions for prairie that I've seen before include "this oyster produces an egg" , "Wide grassy plain" , "Schooners once crossed it" , "Cowboy song: ` `Bury me not on the lone .......' '" , "Continental grassland of N America" .)