Couple going around river, that is plain (7)
I believe the answer is:
prairie
'plain' is the definition.
(type of plain)
'couple going around river that is' is the wordplay.
'couple' becomes 'pr'.
'going around' shows that the letters should be reversed in order.
'river' becomes 'ria' ().
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'ria' written backwards gives 'air'.
'pr'+'air'+'ie'='PRAIRIE'
(Other definitions for prairie that I've seen before include "American plain" , "-- Dog; -- oyster" , "Barren plain" , "Treeless grassy plain, in the old West perhaps" , "Grasslands" .)