Underwater vessel (9)
I believe the answer is:
submarine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underwater vessel</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 submarine that I've seen before include "Under the sea" , "U-boat" , "Under the surface" , "snack" , "Underwater (vessel)" .)