Talk tediously about old whale killer (7)
I believe the answer is:
harpoon
'whale killer' is the definition.
(weapon for killing whales)
'talk tediously about old' is the wordplay.
'talk tediously' becomes 'harp on' (to harp on can mean to talk tediously).
'about' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'harpon' placed around 'o' is 'HARPOON'.
(Other definitions for harpoon that I've seen before include "Weapon for whales" , "Type of spear" , "Whale spear" , "One's cast out at sea" , "Barbed whale-spear" .)