Spear with three prongs (7)
I believe the answer is:
trident
'spear with three prongs' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for trident that I've seen before include "A three-pronged spear as wielded by Neptune" , "Submarine-launched ballistic missile" , "Neptune's spear - missile" , "Red tint (anag.)" , "Neptune's or Britannia's spear" .)