Three-pronged spear (7)
I believe the answer is:
trident
This clue is a double definition.
'three' is the first definition.
(thesaurus)
'pronged spear' is the second definition.
(I've seen this before)
(Other definitions for trident that I've seen before include "Submarine missile" , "Three-pointed spear" , "Britannia's weapon" , "not pike" , "Spear held by Neptune or Britannia" .)