German sub (1-4)
This could be a double definition.
'german' is the first definition.
Although both the answer and definition are adjectives, I don't see how they can define each other.
'sub' is the second definition.
(thesaurus)
(Other definitions for ersatz that I've seen before include "Imitation or substitute, from German" , "Made in imitation, from German" , "'Artificial, not genuine (6)'" , "inferior type" , "Cheap substitute or imitation" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">german sub</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 "About (anag)" , "Submersible warship of WWII" , "German World War One or World War Two submarine" , "Take pool exercise" , "German wartime submarine" .)