Submarine missile (7)
I believe the answer is:
torpedo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">submarine missile</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for torpedo that I've seen before include "Submarine weaponry" , "Trooped off with undersea missile" , "fish you want" , "Sub's missile" , "To wreck" .)