Underwater projectile (7)
I believe the answer is:
torpedo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underwater projectile</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for torpedo that I've seen before include "(Of a project) cause to sink without trace" , "Self-propelled underwater explosive device" , "fish you want" , "Destroy" , "Naval missile" .)