Talk incessantly about old weapon at sea (7)
I believe the answer is:
harpoon
'weapon at sea' is the definition.
(weapon used at sea)
'talk incessantly about old' is the wordplay.
'talk incessantly' becomes 'harp on' (harping on about something is talking about it).
'about' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'harpon' enclosing 'o' is 'HARPOON'.
(Other definitions for harpoon that I've seen before include "word often used in Moby-Dick" , "Barbed whaling spear" , "Whale-catching missile" , "One's cast out at sea" , "Spear with a line attached for fishing" .)