Neptune's spear (7)
I believe the answer is:
trident
'neptune's spear' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for trident that I've seen before include "3-pronged spear" , "Britannia's weapon" , "A three-pronged spear as wielded by Neptune" , "Weapon on submarine" , "Spear held by Neptune or Britannia" .)