Couple crossing river — that is plain (7)
I believe the answer is:
prairie
'plain' is the definition.
(type of plain)
'couple crossing river that is' is the wordplay.
'couple' becomes 'pair' (synonyms).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'pair' enclosing 'r' is 'prair'.
'prair'+'ie'='PRAIRIE'
(Other definitions for prairie that I've seen before include "Large open area of grassland in the US" , "Large tract of grassland in N America" , "Large grassland region" , "Tract of (US) grassland" , "-- Dog; -- oyster" .)