Antitank weapon (7)
I believe the answer is:
bazooka
'antitank weapon' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bazooka that I've seen before include "Anti-tank gun" , "Portable rocket launcher" , "A portable rocket-launcher" , "Portable anti-tank weapon" , "Anti-tank rocket launcher - trumpet-shaped kazoo" .)