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