Marx Brother has no new weapon (7)
I believe the answer is:
harpoon
'weapon' is the definition.
(I've seen this before)
'marx brother has no new' is the wordplay.
'marx brother' becomes 'harpo' (I've seen this before).
'has' means one lot of letters go next to another.
'no' becomes 'O' (resembles 0 - 'no' can mean 'zero').
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'harpo'+'o'+'n'='HARPOON'
(Other definitions for harpoon that I've seen before include "Whale-hunting spear" , "One's cast out at sea" , "Lance" , "Spear with a line attached for fishing" , "Whaler's weapon" .)