Spear an instrument with nothing on! (7)
I believe the answer is:
harpoon
'spear' is the definition.
(I know that harpoon is a type of spear)
'an instrument with nothing on' is the wordplay.
'an instrument' becomes 'harp' (harp is a kind of instrument).
'with' is a charade indicator (letters next to each other).
'nothing' becomes 'O' (O resembles 0).
'harp'+'o'+'on'='HARPOON'
(Other definitions for harpoon that I've seen before include "Type of spear" , "Weapon for whales" , "word often used in Moby-Dick" , "Barbed whaling spear" , "Anti-whale weapon" .)