Undersea vessel (3)
I believe the answer is:
sub
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">undersea vessel</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 sub that I've seen before include "Loan" , "Replacement" , "Briefly, membership fee or stand-in" , "Underwater boat" , "Navy vessel" .)