Underwater weapon (7)
I believe the answer is:
torpedo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underwater weapon</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for torpedo that I've seen before include "Naval missile" , "Such as could sink" , "Self-propelled submarine weapon" , "Explosive underwater weapon" , "attack" .)