Proposals to tour one flat country (6)
I believe the answer is:
plains
'flat country' is the definition.
'proposals to tour one' is the wordplay.
'proposals' becomes 'plans'.
'to tour' means one lot of letters goes inside another (to tour can mean to go around).
'one' becomes 'i' (Roman numeral).
'plans' enclosing 'i' is 'PLAINS'.
(Other definitions for plains that I've seen before include "Prairies" , "Large tracts of level open land" , "Large areas of level country" , "Spinal (anag.)" , "Extensive tracts of level open land" .)