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' is a charade indicator (letters next to each other).
'no' becomes 'o' (resembles 0 - 'no' can mean 'zero').
'new' becomes 'n'.
'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" .)