Vessel crossing river -- 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 atomic' is the wordplay.
'vessel' becomes 'can' (type of container).
'crossing' is an insertion indicator.
'river' becomes 'tamar' (I've seen this before).
'atomic' becomes 'a' (eg in A-test, atomic test).
'tamar'+'a'='tamara'
'can' placed around 'tamara' is 'CATAMARAN'.
(Other definitions for catamaran that I've seen before include "Vessel with twin hulls" , "Two-hulled boat" , "raft of logs" , "Vessel with paralleltwin hulls" , "Quarrelsome woman" .)