Submersible warship (9)
I believe the answer is:
submarine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">submersible warship</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 submarine that I've seen before include "snack" , "Under the sea" , "Captain Nemo's Nautilus" , "... underwater craft" , "Underwater (vessel)" .)