Stretch vessels in river (7)
I believe the answer is:
expanse
'stretch' is the definition.
(I've seen this before)
'vessels in river' is the wordplay.
'vessels' becomes 'pans' (pans are vessels or containers).
'in' means one lot of letters goes inside another.
'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" .)