Couple getting right inside – that is plain (7)
I believe the answer is:
prairie
'is plain' is the definition.
(type of plain)
'couple getting right inside that' is the wordplay.
'couple' becomes 'pair' (synonyms).
'getting' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'inside that' becomes 'ie' (I can't explain this - if you can you should give a lot more credence to this answer).
'pair' going around 'r' is 'prair'.
'prair'+'ie'='PRAIRIE'
Can you help me to learn more?
(Other definitions for prairie that I've seen before include "Extensive US grassland" , "Tract of (US) grassland" , "American plain" , "Continental grassland of N America" , "Large treeless tract of grassland" .)