Neptune's spear was a missile! (7)
I believe the answer is:
trident
This clue is a double definition.
'neptune's spear' is the first definition.
(I've seen this in another clue)
'a missile' is the second definition.
(UK nuclear weapons system)
'was' acts as a link.
(Other definitions for trident that I've seen before include "Neptune's spear - missile" , "spear carried at sea" , "3-pronged spear" , "A three-pronged spear as wielded by Neptune" , "Fork perhaps" .)