Vessels seen in river region (7)
I believe the answer is:
expanse
'region' is the definition.
'vessels seen in river' is the wordplay.
'vessels' becomes 'pans' (I've seen this in other clues).
'seen in' is an insertion indicator.
'river' becomes 'exe' (River Exe).
'pans' put inside 'exe' is 'EXPANSE'.
(Other definitions for expanse that I've seen before include "Wide area of open land or space" , "reach" , "Range - spread" , "Swathe" , "considerable area" .)