Grassland that is south of river toured by twins, say? (7)
I believe the answer is:
'grassland' is the definition.
(I know this)
'that is south of river toured by twins say?' is the wordplay.
'that is' becomes 'i.e.' (short for Latin 'id est', 'that is').
'south of' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation for river).
'toured by' indicates putting letters inside (tour can mean to go around).
'twins say?' becomes 'pair' (a pair of twins).
'r' going inside 'pair' is 'prair'.
'ie' put after 'prair' is 'PRAIRIE'.
(Other definitions for prairie that I've seen before include "Open grassland of North America", "-- Dog; -- oyster", "Large grassland region", "Large tract of grassland in N America", "Open grassland in the USA".)