Brother unable to speak regarding weapon (7)
I believe the answer is:
harpoon
'regarding weapon' is the definition.
'harpoon' can be an answer for 'weapon' (I've seen this before). I am unsure of the 'regarding' bit.
'brother unable to speak' is the wordplay.
'brother' becomes 'harpo' (I've seen this before).
'unable to speak' becomes 'on' (I am not sure about this - if you are sure you should believe this answer much more).
'harpo'+'on'='HARPOON'
Can you help me to learn more?
(Other definitions for harpoon that I've seen before include "Spear with a line attached for fishing" , "Spear for old whaler" , "Barbed whale-spear" , "Whaling weapon" , "Type of spear" .)