Boat, vessel crossing a river (9)
I believe the answer is:
catamaran
'boat' is the definition.
(I know that catamaran is a type of boat)
'vessel crossing a river' is the wordplay.
'vessel' becomes 'can' (type of container).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'river' becomes 'tamar' (river in the southwest of England).
'a'+'tamar'='atamar'
'can' placed around 'atamar' is 'CATAMARAN'.
(Other definitions for catamaran that I've seen before include "Two-hulled boat" , "Twin-hulled craft" , "Twin-hulled sailing boat" , "Boat with two hulls" , "raft of logs" .)