Three-pronged spear (7)
I believe the answer is:
trident
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">three</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">pronged spear</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(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" .)