Stretch of Devon river full of vessels (7)
I believe the answer is:
expanse
'stretch' is the definition.
(thesaurus)
'devon river full of vessels' is the wordplay.
'devon river' becomes 'exe' (I've seen this before).
'full of' is an insertion indicator.
'vessels' becomes 'pans' (I've seen this in other clues).
'exe' going around 'pans' is 'EXPANSE'.
'of' is the link.
(Other definitions for expanse that I've seen before include "considerable area" , "Wide area of open land or space" , "reach" , "Wide continuous area" , "Range - spread" .)