Plain couple that is welcoming affair, finally (7)
I believe the answer is:
prairie
'plain' is the definition.
(thesaurus)
'couple that is welcoming affair finally' is the wordplay.
'couple' becomes 'pair' ('pair' can be a synonym of 'couple'**).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'welcoming' means one lot of letters goes inside another (in sense of 'inviting in').
'finally' indicates one should take the final letters.
The last letter of 'affair' is 'r'.
'pair'+'ie'='pairie'
'pairie' enclosing 'r' is 'PRAIRIE'.
(Other definitions for prairie that I've seen before include "Cowboy song: ` `Bury me not on the lone .......' '" , "Open grassland in the USA" , "Continental grassland of N America" , "Open grassland of North America" , "Grasslands" .)