Vessels in river region (7)
I believe the answer is:
expanse
'region' is the definition.
'vessels in river' is the wordplay.
'vessels' becomes 'pans' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'river' becomes 'exe' (I've seen this before).
'pans' put within 'exe' is 'EXPANSE'.
(Other definitions for expanse that I've seen before include "Swathe" , "considerable area" , "Range - spread" , "reach" , "Wide continuous area" .)