Vessel contains bad heavy weapons (9)
I believe the answer is:
artillery
'heavy weapons' is the definition.
(I've seen this before)
'vessel contains bad' is the wordplay.
'vessel' becomes 'artery' (artery is a kind of vessel).
'contains' means one lot of letters goes inside another.
'bad' becomes 'ill' (associated in meaning).
'artery' enclosing 'ill' is 'ARTILLERY'.
(Other definitions for artillery that I've seen before include "Large pieces" , "Heavy guns" , "Large-calibre guns" , "Guns, cannons etc" , "Missile-launchers and the like" .)