A three-pronged spear (7)
I believe the answer is:
trident
I believe this clue is a double definition.
'a 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 "Red tint? (anagram)" , "Neptune's or Britannia's spear" , "Weapon on submarine" , "3-pronged spear" , "Three-pointed spear" .)