Vessel crossing river with atomic boat (9)
I believe the answer is:
catamaran
'boat' is the definition.
(I know that catamaran is a type of boat)
'vessel crossing river with atomic' is the wordplay.
'vessel' becomes 'can' (type of container).
'crossing' indicates putting letters inside.
'river' becomes 'tamar' (river in the southwest of England).
'with' is a charade indicator (letters next to each other).
'atomic' becomes 'a' (eg in A-test, atomic test).
'tamar' after 'a' is 'atamar'.
'can' enclosing 'atamar' is 'CATAMARAN'.
(Other definitions for catamaran that I've seen before include "Boat with two hulls" , "Twin- hulled vessel" , "Twin-hulled sailing boat" , "Boat with twin, parallel hulls" , "One vessel" .)