Couple eating last of cracker that is plain (7)
I believe the answer is:
prairie
'plain' is the definition.
(type of plain)
'couple eating last of cracker that is' is the wordplay.
'couple' becomes 'pair' (synonyms).
'eating' indicates putting letters inside.
'last of' says to take the final letters.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
The final letter of 'cracker' is 'r'.
'pair' placed around 'r' is 'prair'.
'prair'+'ie'='PRAIRIE'
(Other definitions for prairie that I've seen before include "Schooners once crossed it" , "Barren plain" , "Treeless grassy plain, in the old West perhaps" , "Large open area of grassland in the US" , "Wide grassy plain" .)