Stretch vessels in river (7)
I believe the answer is:
expanse
'stretch' is the definition.
(I've seen this in another clue)
'vessels in river' is the wordplay.
'vessels' becomes 'pans' (pans are vessels or containers).
'in' is an insertion indicator.
'river' becomes 'exe' (River Exe).
'pans' inserted into 'exe' is 'EXPANSE'.
(Other definitions for expanse that I've seen before include "Wide continuous area" , "Swathe" , "considerable area" , "Stretch - extent" , "reach" .)