Area with river occupied by vessels (7)
I believe the answer is:
expanse
'area' is the definition.
('expanse' can be a synonym of 'area')
'river occupied by vessels' is the wordplay.
'river' becomes 'exe' (I've seen this before).
'occupied by' means one lot of letters goes inside another.
'vessels' becomes 'pans' (I've seen this in other clues).
'exe' going around 'pans' is 'EXPANSE'.
'with' acts as a link.
(Other definitions for expanse that I've seen before include "Swathe" , "Wide continuous area" , "Stretch - extent" , "Range - spread" , "Wide area of open land or space" .)