Stretch of river in Devon full of vessels (7)
I believe the answer is:
expanse
'stretch' is the definition.
(thesaurus)
'river in devon full of vessels' is the wordplay.
'river in devon' becomes 'exe' (I've seen this before).
'full of' indicates putting letters inside.
'vessels' becomes 'pans' (I've seen this in other clues).
'exe' placed around 'pans' is 'EXPANSE'.
'of' acts as a link.
(Other definitions for expanse that I've seen before include "Range - spread" , "Swathe" , "Wide continuous area" , "Stretch - extent" , "reach" .)