Width of river accommodating vessels (7)
I believe the answer is:
expanse
'width' is the definition.
(thesaurus)
'river accommodating vessels' is the wordplay.
'river' becomes 'exe' (River Exe).
'accommodating' is an insertion indicator.
'vessels' becomes 'pans' (I've seen this before).
'exe' placed around 'pans' is 'EXPANSE'.
'of' acts as a link.
(Other definitions for expanse that I've seen before include "Range - spread" , "considerable area" , "Swathe" , "Wide area of open land or space" , "reach" .)