River vessel with little old Scot at the stern (7)
I believe the answer is:
potomac
'river' is the definition.
(I know that Potomac is a river)
'vessel with little old scot at the stern' is the wordplay.
'vessel' becomes 'pot' (type of container).
'with' means one lot of letters go next to another.
'little old' becomes 'o' (abbreviation for old).
'scot' becomes 'mac' (many Scottish surnames begin with Mac).
'at the stern' means one lot of letters go next to another (stern can mean the back of a boat).
'pot'+'o'+'mac'='POTOMAC'
(Other definitions for potomac that I've seen before include "to which banks in Washington have access" , "Water flowing" , "Flower" , "Capital outflow" , "Washington's river" .)