German sub (6)
This clue could be a double definition.
'german' is the first definition.
Although both the answer and definition are adjectives, I cannot understand how they can define each other.
'sub' is the second definition.
(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" .)
'german sub' is the definition.
(I've seen this before)
This is all the clue.
(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" .)