Flat-bottomed vessels in Devon river region (7)
I believe the answer is:
expanse
'region' is the definition.
'flat-bottomed vessels in devon river' is the wordplay.
'flat-bottomed vessels' becomes 'pans'.
'in' means one lot of letters goes inside another.
'devon river' becomes 'exe' (I've seen this before).
'pans' inserted within 'exe' is 'EXPANSE'.
(Other definitions for expanse that I've seen before include "Stretch - extent" , "considerable area" , "Wide continuous area" , "Range - spread" , "Wide area of open land or space" .)