Instrument having nothing on a barbed spear (7)
I believe the answer is:
harpoon
'a barbed spear' is the definition.
(I've seen this before)
'instrument having nothing on' is the wordplay.
'instrument' becomes 'harp' (harp is a kind of instrument**).
'having nothing' becomes 'o' (I've seen this in other clues).
'harp'+'o'+'on'='HARPOON'
(Other definitions for harpoon that I've seen before include "Whale-catching missile" , "whaling equipment" , "One's cast out at sea" , "Spear for old whaler" , "Spear with a line attached for fishing" .)