Vessel crossing river in southwest — 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 in southwest atomic' is the wordplay.
'vessel' becomes 'can' (type of container).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'river in southwest' becomes 'tamar' (I can't explain this - if you can you should give a lot more credence to this answer).
'atomic' becomes 'a' (eg in A-test, atomic test).
'tamar'+'a'='tamara'
'can' placed around 'tamara' is 'CATAMARAN'.
Can you help me to learn more?
(Other definitions for catamaran that I've seen before include "Vessel with paralleltwin hulls" , "Sailing vessel" , "Twin-hulled craft" , "raft of logs" , "Twin-hulled vessel" .)