Romeo will be enthralled by partner — that is plain (7)
I believe the answer is:
prairie
'plain' is the definition.
(type of plain)
'romeo will be enthralled by partner that is' is the wordplay.
'romeo will' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'be enthralled by' indicates putting letters inside ('enthral' can mean 'capture').
'partner' becomes 'pair' (both can mean to form a pairing).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'r' going inside 'pair' is 'prair'.
'prair'+'ie'='PRAIRIE'
Can you help me to learn more?
(Other definitions for prairie that I've seen before include "Grasslands" , "Broad rolling tract of land in the US" , "Large grassland region" , "Large open area of grassland in the US" , "North American grassland" .)