Marx Brother talking about whaler's weapon (7)
I believe the answer is:
harpoon
'whaler's weapon' is the definition.
(I've seen this before)
'marx brother talking about' is the wordplay.
'marx brother' becomes 'harpo' (I've seen this before).
'talking about' becomes 'on' (on can mean relating to or about a certain subject).
'harpo'+'on'='HARPOON'
(Other definitions for harpoon that I've seen before include "Barbed whale-spear" , "Whaler's weapon" , "Whale-hunting spear" , "Spear with a line attached for fishing" , "Barbed whaling spear" .)