Persist in talking about old weapon (7)
I believe the answer is:
harpoon
'weapon' is the definition.
(I've seen this before)
'persist in talking about old' is the wordplay.
'persist' becomes 'harp' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'talking about' becomes 'on' (on can mean relating to or about a certain subject).
'old' becomes 'o' (abbreviation).
'on' placed around 'o' is 'oon'.
'harp'+'oon'='HARPOON'
Can you help me to learn more?
(Other definitions for harpoon that I've seen before include "One's cast out at sea" , "Barbed whaling spear" , "Whaler's missile" , "Whale-catching missile" , "word often used in Moby-Dick" .)