Stretch of river occupied by vessels (7)
I believe the answer is:
expanse
'stretch' is the definition.
(thesaurus)
'river occupied by vessels' is the wordplay.
'river' becomes 'exe' (I've seen this before).
'occupied by' means one lot of letters goes inside another.
'vessels' becomes 'pans' (I've seen this in other clues).
'exe' enclosing '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" , "Swathe" , "Range - spread" , "reach" .)