Wartime submarine (1-4)
I believe the answer is:
u-boat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">wartime submarine</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 u-boat that I've seen before include "German World War One or World War Two submarine" , "About (anag)" , "Submersible warship armed with torpedoes" , "German WW 2 sub" , "Submersible warship of WWII" .)