River vessel, ordinary and waterproof (7)
I believe the answer is:
potomac
'river' is the definition.
(I've seen this before)
'vessel ordinary and waterproof' is the wordplay.
'vessel' becomes 'pot' (type of container).
'ordinary' becomes 'O' (abbreviation e.g. O-Levels).
'and' means one lot of letters go next to another.
'waterproof' becomes 'mac' (mac is a kind of waterproof).
'pot'+'o'+'mac'='POTOMAC'
(Other definitions for potomac that I've seen before include "to which banks in Washington have access" , "River of Washington DC" , "Flower" , "Washington's river" , "Water flowing" .)