Flat-bottomed vessels in river region (7)
I believe the answer is:
expanse
'region' is the definition.
'flat-bottomed vessels in river' is the wordplay.
'flat-bottomed vessels' becomes 'pans'.
'in' means one lot of letters goes inside another.
'river' becomes 'exe' (River Exe).
'pans' going into 'exe' is 'EXPANSE'.
(Other definitions for expanse that I've seen before include "Wide continuous area" , "reach" , "Range - spread" , "Wide area of open land or space" , "Swathe" .)