Couple crossing river that is in grassland (7)
I believe the answer is:
prairie
'grassland' is the definition.
(prairie is a kind of grassland)
'couple crossing river that is' is the wordplay.
'couple' becomes 'pair' ('pair' can be a synonym of 'couple').
'crossing' indicates putting letters inside.
'river' becomes 'r'.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'pair' going around 'r' is 'prair'.
'prair'+'ie'='PRAIRIE'
'in' acts as a link.
(Other definitions for prairie that I've seen before include "Schooners once crossed it" , "Open grassland in the USA" , "Large open area of grassland in the US" , "Open grassland of North America" , "Cowboy song: ` `Bury me not on the lone .......' '" .)