German submarine (1-4)
I believe the answer is:
u-boat
'german submarine' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for u-boat that I've seen before include "German submarine of WW2" , "About (anag)" , "German WW 2 sub" , "German World War One or World War Two submarine" , "Sea vessel" .)