Stretch to find 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' (I've seen this before).
'in' indicates putting letters inside.
'river' becomes 'exe' (River Exe).
'pans' going into 'exe' is 'EXPANSE'.
'to find' is the link.
(Other definitions for expanse that I've seen before include "Wide continuous area" , "considerable area" , "Stretch - extent" , "Swathe" , "Range - spread" .)